Single Storey Extensions
The most common brief in Colchester, and the one where the ground under the garden decides more of the price than anything else.
Fig. 01
A single storey extension is the most common thing we are asked to price in Colchester, and the one where two quotes for the apparently identical job can differ by twenty thousand pounds. This page explains where that difference comes from.
What a single storey extension actually is
One floor, added to the ground floor of the house, with its own foundations and its own roof.
It sounds obvious, but the definition matters because it determines what rules apply. A single storey extension is a permanent structure with foundations designed for the ground it sits on, walls built to current insulation standards, a roof designed to shed water and carry snow, and a connection into the existing house that involves removing part of a load bearing wall.
That distinguishes it from a conservatory, which can be exempt from most Building Regulations on three conditions and no more: it is at ground level, its floor area does not exceed 30 square metres, and its glazing meets the Part K safety requirements. A separate rule then holds the energy requirements off it, but only while it stays genuinely separated from the house by external quality walls, doors or windows and is heated, if at all, on its own controls. Lose either of those and it is an extension. It also distinguishes it from a garden room, which is a separate outbuilding under different permitted development rules.
Most single storey extensions in Colchester are rear extensions: straight off the back of the house into the garden. Side extensions go out into a driveway or side passage. Side return extensions fill the narrow alley beside a Victorian terrace’s rear outrigger. A wrap around does both. The construction is much the same; what changes is the planning position and the access.
The reason people build them is almost always the same. The kitchen is too small, it is separate from where the family actually sits, and the back of the house is dark. A single storey extension solves all three at once, which is why it is the default answer rather than going up, and why it is worth doing properly.
Will you need planning permission
Usually not. But “usually” hides several things that catch people out.
Most single storey rear extensions fall under permitted development, which means planning permission has already been granted in general terms and you do not need to apply. The rights are national and specific, and they come with conditions on depth, height, materials and boundary position.
Depth is measured from the original rear wall of the house. Original means as it was built, or as it stood in 1948 if it is older than that. This is the single most common misunderstanding we encounter. If a previous owner added a small extension in the 1990s, that depth has already been used, and it counts against you even though you did not build it. We check the planning history before anyone draws anything.
Broadly, a detached house is allowed to go deeper than a semi or a terrace. Both can go further under the Larger Home Extension prior approval route. That is not a planning application, but it is not nothing either: you notify the council, the council writes to the adjoining neighbours, and if a neighbour objects within the consultation period the council decides whether the impact is acceptable. It runs to 42 days.
Height is capped, and there is a lower cap on eaves height where the extension comes within two metres of a boundary. On a narrow terraced plot that limit does real work and often determines the roof design.
Permitted development does not apply at all to flats and maisonettes. Listed status works differently: it does not by itself remove permitted development rights, but listed building consent is required separately, under separate legislation, for anything affecting the building’s special interest, inside as well as out. Carrying out that work without consent is a criminal offence rather than a planning irregularity, so on a listed building the consent question comes first whatever the permitted development answer is. It is restricted in conservation areas, where cladding and side extensions generally lose their permitted status. An Article 4 direction can remove it for a specific area, and a newer estate may have lost it by condition. And covenants on the title are a separate matter from planning entirely: they are private law, the council will not check them, and they can still stop you.
Even where the work is clearly permitted development, apply for a Lawful Development Certificate. It costs a fraction of a planning fee and it is the document that proves the extension was lawful when a buyer’s solicitor asks in ten years. Without it you are relying on memory.
The limits summarised here change from time to time and how they apply depends on your specific property. Treat this as an explanation of the system rather than confirmation of what you can build. We check the position on your address as the first thing we do.
Will you need planning permission?
Four questions, an honest answer, and nothing sent anywhere. This runs entirely in your browser.
What are you extending?
Permitted development rights for extensions apply to houses. Flats and maisonettes are a different route entirely.
Is the property listed?
Listed building consent is a separate decision from planning permission, and work without it is a criminal offence rather than a planning irregularity.
Has the house been extended before?
Depth is measured from the ORIGINAL rear wall, so a previous owner's extension has already used part of your allowance. This catches almost everybody.
Is it in a conservation area?
Colchester has a number of them, including the historic core, the Dutch Quarter, part of Lexden and several village centres. The boundaries rarely follow the roads you would expect.
Answer the four questions for an indication
Whatever the answer, Building Regulations approval is always needed. We check the planning history, the designation and the deeds on your address before anyone draws anything, because it decides the design rather than the other way round.
This is an indication based on four questions, not advice on your property. Planning rules change and the position depends on your specific address, its planning history and its designation.
Foundations, and why Essex clay sets your price
The biggest cost variable on the whole job is invisible until somebody digs.
Most of the ground around Colchester is shrinkable clay. Clay takes on water and swells, then dries out and shrinks. Near the surface that movement is seasonal and significant. A foundation founded inside that zone moves, and because the existing house is founded deeper and does not move with it, the extension cracks away from the house. It shows up at the junction, usually in the second or third summer, and it is expensive to put right.
The answer is to found below the zone of seasonal movement. On an open site with no trees nearby that is typically around 900mm, though the exact figure depends on the plasticity of the clay itself.
Trees change everything. A mature oak, poplar, willow or elm draws an enormous volume of water out of the ground across a summer, and its zone of influence extends well beyond the spread of its canopy. Foundation depth is then set by the species, its mature height and its distance from the extension. In the worst cases you are down at two and a half metres or more, or moving to a piled foundation with a suspended reinforced slab and a void former beneath it.
Removing the tree is not the shortcut it appears to be. Ground that has been dried by a tree for forty years will slowly take that water back once the tree is gone, and swell. That is heave, it acts upwards, and it does more damage than settlement does. Where a tree has been recently removed, the foundation design has to account for it.
This is where cheap quotes come from. A price based on a one metre dig and a price based on a two and a half metre dig next to a protected oak are not quotes for the same job. Ask any builder pricing your extension what foundation depth they have allowed and what happens if it needs to go deeper. If the answer is vague, the number is not a price, it is a deposit.
The structural opening
Connecting the new space to the old is the structural event of the whole job.
There is little point building a beautiful extension and leaving a doorway into it. The value comes from removing the old rear wall so the two spaces read as one. That wall is carrying floor joists and often the roof, and the load has to be picked up.
It goes into a steel beam. The beam bears onto padstones at each end, and the padstones sit on masonry that has to be capable of taking a concentrated point load it was never designed for. The structural engineer sizes the beam, specifies the padstones, and checks the masonry below them. Where that masonry is not adequate, the load has to be taken down to foundation level, which sometimes means a pier or a new pad.
Everyone wants a wide opening with a slim beam. The depth of a steel is a function of span and load, and there is no negotiating with it. On a wide span you can sometimes use a deeper beam hidden within the floor zone above, or two beams, or a different structural arrangement. What you cannot do is ask for a smaller steel and expect the physics to cooperate.
The temporary support while the wall is out is its own piece of design and its own risk. Needles and props, correctly positioned and correctly loaded. This is the part of the job where cutting corners does not show up as a crack, it shows up as a collapse.
Want this checked on your actual address?
Planning history, designation and deeds. We look at all three before anyone draws anything, and it costs you nothing.
Roof options
Flat or pitched, and warm deck or cold deck. Both choices have consequences you live with.
A flat roof is not actually flat. It has a fall built into it, and modern single ply membranes and GRP systems are reliable and long lived when detailed properly. Flat roofs suit deep extensions where a pitched roof would either be too tall for permitted development or would block light from the first floor windows above.
The detail that matters is warm deck versus cold deck. In a warm deck the insulation sits above the structural deck, so the structure stays at internal temperature and there is nowhere for condensation to form. In a cold deck the insulation is between the joists and the void above it must be ventilated. Cold decks are cheaper and they fail more often, because the ventilation gets blocked or was never adequate. We build warm decks.
A pitched roof reads better on some houses, particularly period property and anything in a conservation area, and it gives you the option of a vaulted ceiling inside. The cost is height: the ridge has to go somewhere, and on a rear extension it frequently ends up under a first floor window, which is both a planning problem and a daylight problem. On a bungalow there is no window above.
Rooflights are the single most effective thing you can add to either. A rooflight delivers substantially more usable daylight than a vertical window of the same area, and set back from the glazed rear wall they bring light into the middle of the plan where it is actually needed.
Glazing, light and the room you end up with
More glass is not the same as more light, and it is definitely not the same as a better room.
Part L limits the total area of windows, doors and rooflights in an extension to a proportion of its floor area, with an allowance for the openings in the existing wall you are covering over. You can exceed it, but you then have to compensate elsewhere with better insulation or better glazing units and demonstrate the extension performs as well overall. That is a design decision with a cost, not a free choice.
Overheating matters on any highly glazed extension facing anywhere near south. Approved Document O sets overheating requirements for new dwellings rather than extensions, so it does not formally apply, but the problem is real whether or not a regulation names it. A wall of bifolds under a flat roof facing south west will be unusable on a July afternoon unless something has been done: solar control glazing, an overhang, and openable area positioned so air crosses the room rather than stirring in one corner.
Orientation should drive the design. North facing rear elevations can take as much glass as you can justify, because the light is even and it never overheats. South and west facing ones want less glass at the end and more light from above.
The mistake we are asked to fix most often is the middle room. Extend across the full width of a terrace or a narrow semi and the old middle room loses its only window and becomes a dark corridor between the front room and the new one. That is entirely avoidable at drawing stage, with a light well, a set back, or rooflights over what used to be the outside wall. It is not avoidable afterwards.
The same wall, before and after
Drag to reveal. A real job in the Colchester area, photographed as it went. No staging and no borrowed images.
During
Finished
Side extensions and side returns
Same construction, different planning position, and a very different site to work on.
A side extension goes out into a driveway or side passage rather than back into the garden. Under permitted development it is limited to half the width of the original house and to a single storey, with a height cap. In a conservation area it generally loses permitted development status altogether, so it becomes an application.
The practical constraint is usually not planning but access. Build across the side passage and you lose the route from the front of the house to the back garden, permanently. On a semi detached house that frequently means the only access to the garden afterwards is through the house, which matters for bikes, bins, garden waste and any future building work. It is worth thinking about before rather than discovering it after.
A side return extension is the Victorian and Edwardian terrace version: the narrow alley beside the rear outrigger, roofed and built out to the boundary so the kitchen becomes the full width of the plot. It is the single most effective change you can make to a terraced ground floor, because the rooms it produces are wide rather than long.
Two things make side returns harder than they look. The first is that you are building right up to a boundary wall you do not own, which brings the Party Wall Act into play as a certainty rather than a possibility, and means the wall has to be assessed before anything is designed. The second is light: a side return is bounded by a wall a metre or two away on one side, so vertical glazing at the end does very little. The light has to come from above, which is why almost every good side return has a run of rooflights along the boundary.
Access on a terrace is the other cost. Without a side gate, every barrow of muck out and every load of material in goes through the hallway on boards. That is slower, it needs more protection, and it is properly reflected in a realistic price.
Insulation, heating and how the room will feel
A cold extension is a room nobody uses, and it is entirely a specification decision.
Part L sets minimum insulation values for the new walls, floor and roof, and modern minimums are considerably better than the house you are attaching to. That produces an odd effect people do not expect: the new extension is often the warmest part of the house, and the old rooms feel worse by comparison.
The floor is where corners get cut most often, because nobody sees it. Insulation under a ground bearing slab is cheap at the time and impossible to add later. Get it right.
Thermal bridging is the detail that decides whether the extension performs the way the calculation says it will. Cold bridges at the wall to floor junction, around openings, and at the eaves are where heat leaves and where condensation forms on the inside face in winter. Insulated cavity closers around every opening, insulation carried down past the slab edge, and continuity at the roof junction. None of it is visible in the finished room and all of it is the difference between a warm extension and a cold one.
Heating is a genuine design question rather than an afterthought. Adding twenty square metres of heated space to a house means the existing boiler is now serving a larger load, and on an older system that is worth checking rather than assuming. Underfloor heating suits an extension well because the floor is being built from scratch anyway, and it pairs sensibly with a large glazed area where a radiator would have nowhere sensible to go. If it is going in, it is decided before the slab, not after.
Five things you can check yourself.
Trust on a building job should be verifiable, not asserted. Every one of these can be confirmed without taking our word for it.
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01
Real company
Essex Probuild Ltd, company 15979928, registered in West Bergholt. Checkable on Companies House in a minute.
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Real cover
£2m public liability cover in place. Evidence available on request rather than a badge on a website.
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Real reviews
4.6 out of 5 on Google from 10 reviews, shown as they were left. We do not curate them.
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Real photographs
Every project image on this site is a job we built. No stock, and no fabricated before shots.
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Real accountability
We notify Colchester City Council Building Control and manage the inspections through to your completion certificate.
Materials, and matching the existing house
Match it properly or contrast it deliberately. The failure is trying to match and missing.
Brick is the difficulty. A house built in the 1930s has bricks that no longer exist in that form, and forty years of weathering on top. A new brick chosen from a sample board in good light will not look like the wall it is next to. Where matching genuinely matters, particularly on a visible elevation or in a conservation area, reclaimed brick or a specific blend is worth the cost.
Where matching is not realistic, the better answer is usually deliberate contrast. Cladding, render or a darker brick, in a form that reads as a clearly modern addition rather than a failed impersonation. Planning officers tend to prefer that too, because an honest addition is easier to assess than a pastiche.
The junction is what sells it either way. Where the new roof meets the existing wall, where the new brickwork meets the old, and where the eaves line runs through, are the details a passer by reads without knowing they are reading them. A cavity tray and a properly formed abutment at that junction is also, incidentally, the detail that stops water tracking into the house.
Drainage
The drain under the lawn is a design constraint, and occasionally a planning one.
If the extension is built over or within three metres of a public sewer, you need a build over agreement from Anglian Water. That is a separate application with its own timescale. It can require a CCTV survey of the drain, a foundation design that bridges over it rather than loading it, or in some cases a diversion.
Private drains within your own boundary are more flexible but still have to comply with Part H. An inspection chamber cannot simply be built over and forgotten. Where a manhole falls inside the new footprint it is either moved or fitted with a sealed double sealed cover, and access has to be maintained.
Surface water is the other half of it. A new roof plus a new patio adds run off that has to go somewhere, and soakaways in clay work poorly. This gets designed rather than assumed.
What might it cost?
A realistic range rather than a single number, with what it excludes shown as prominently as what it includes.
£44k to £60k
- VAT
- Kitchen and appliances
- Flooring
- Decoration
- Drawings and calculations
- Planning and Building Control fees
- Party wall surveyor
- Service diversions
Indicative only. The biggest variable around Colchester is foundation depth, because most of the ground is shrinkable clay and mature trees are common. We price the job after seeing it.
Get a real price
Building Regulations
Separate from planning, and the part that decides whether the building actually works.
Planning is about whether you may build. Building Regulations are about how it is built. Permitted development work still needs Building Regulations approval.
On a single storey extension the Approved Documents that apply are A for structure, B for fire safety including escape windows, C for damp proofing and cavity trays, F for ventilation, H for drainage, K for guarding and glazing, L for insulation and glazed area, and P for electrical safety, since new circuits in a kitchen or an extension are notifiable work and have to be certified by a competent person.
There are two routes. A full plans submission means the drawings and the engineer’s calculations are checked before you start and you receive a decision notice. A building notice means you tell the council you are starting and they inspect as you go. Full plans takes longer at the front and removes almost all the risk of an expensive disagreement partway up a wall. On anything involving a structural opening we use full plans.
We notify Colchester City Council Building Control and we manage the inspections. At completion you get a completion certificate, which you should keep with your deeds.
What a single storey extension costs
Ranges with their basis stated, because a single number from someone who has not seen your house is worthless.
Costs move with materials, labour and specification, so treat this as indicative at the review date at the foot of this page rather than as a quotation.
As a broad guide, a straightforward single storey extension on reasonable ground with decent access typically runs £2,200 to £3,000 per square metre for the shell and a standard finish. Difficult access, deep foundations, wide structural openings, extensive glazing or a high specification push above that, and there is no upper limit on the last of those.
That range normally includes groundworks and foundations to the specified depth, drainage connections, structure and the steel, roof, windows and external doors, plastering, first and second fix carpentry, electrics and plumbing to a standard specification, and making good where the new work meets the old.
It normally excludes, and you should budget separately for: VAT, kitchen units and appliances, flooring, decoration, professional fees for drawings and structural calculations, planning and Building Control fees, party wall surveyor fees if a neighbour dissents, service diversions, and anything genuinely unforeseeable found once the ground is open.
The variables that move the number most, in order, are foundation depth, access, structural span, glazing specification and roof type. An identical extension on a terraced house where every barrow load comes through the hallway costs meaningfully more than one on a house with a side gate, and that has nothing to do with the extension itself.
Get a real number, not a range.
We come out, look at the ground, the access and the structure, and price the job we have actually seen. The survey and the quotation are free.
How long it takes
Two months of paperwork you can overlap, then two to four months on site.
Before anything starts: a householder planning application has a statutory determination period of eight weeks from validation. Prior approval runs to 42 days. Building Regulations full plans approval runs alongside. Party wall notices, where they apply, run one or two months depending on the section, and those periods are fixed in law, and the period runs from service rather than from when everyone becomes agreeable. It can be shortened only one way: an adjoining owner who consents in writing allows the work to start sooner. What you cannot do is proceed unilaterally because nobody has objected.
These overlap, and we run them in parallel where we can, but the front end of a single storey extension is realistically two months and sometimes more.
On site, a straightforward single storey extension is commonly eight to fourteen weeks from breaking ground to completion. Deep foundations add time. Weather affects groundworks and roofing, and a wet February moves a programme.
The disruptive phase is the beginning, when the ground is open and the opening is being formed. Once the shell is weathertight it becomes much more tolerable. The day the back wall comes out is planned with you rather than announced.
What we are asked to put right
The recurring failures, so you can avoid buying them.
Foundations too shallow for the clay, cracking at the junction with the house within two summers. Cold deck flat roofs with blocked or absent ventilation, condensing on the underside of the deck and rotting the joists. A steel installed without padstones, crushing the masonry beneath its bearings. Cavity trays omitted where the new roof abuts the existing wall, so water tracks through into the house above the junction. A manhole built over with no access. And the dark middle room, which is not a defect at all, just a design nobody thought about.
None of these are exotic. All of them are cheaper to avoid than to fix.
Fig. 01
What actually happens, in order
Three of these eight stages happen before anybody digs anything. Get the order wrong and you pay to redo work.
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Survey
We look at the ground, the trees, the access, the existing structure and where the drains actually run.
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Drawings
A technician produces existing and proposed plans and elevations to a standard the council will accept.
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Permissions
Permitted development, prior approval or a full application. We submit it and handle the correspondence.
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Structure
The engineer calculates the beams, the padstones and the foundation depth for your ground.
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Groundworks
Dig, concrete and drainage. The messiest phase and the one that decides whether the rest stands still.
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Structure up
Walls, steel into the opening, roof on, windows and doors in. The day the back wall comes out is planned with you.
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Fit out
First fix, plaster, second fix. From here it stops being a building site and starts being a room.
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Certificate
Final inspection and your Building Control completion certificate. Keep it with the deeds.
Questions we get asked
Can I stay in the house while it is built?
Almost always, yes. A single storey rear extension is built outside the existing wall, so for most of the programme the house is intact behind it. The exception is the period around forming the structural opening, when the back of the house is temporarily supported and open. That is a matter of days rather than weeks, and we plan it with you. Where the extension involves the kitchen, we tell you before you commit how long you will be without one.
Do I need an architect?
Not necessarily. For a straightforward extension a competent architectural technician produces drawings that are entirely adequate for planning and for building from, at a fraction of the cost. An architect earns their fee where the design problem is genuinely difficult, on a listed building, or where you want something out of the ordinary. We are builders, not architects, and we will tell you honestly which of those your job is.
Can I do some of the work myself to save money?
Decoration, flooring and landscaping afterwards, easily, and plenty of clients do. Anything structural, notifiable electrical work, or anything that has to be inspected by Building Control, no. It confuses the responsibility for the certificate and it usually costs more to unpick than it saved.
What happens if you find something unexpected?
We stop, show you what we have found, explain the options and price the change before doing anything. You approve variations in writing. We do not carry out extra chargeable work and present it at the end.
How we do it
One contract, one contact, and the paperwork is our problem.
Our team has 25+ years in the trade and comes out. The first visit is a survey, not a sales call: we look at the ground, the trees, the access, the existing structure, where the drains run and what you are actually trying to achieve.
Most people who call us already have drawings and permission and want a builder to price the work and build it. That is the ordinary job here, and we are glad to work to somebody else’s plans. Where the drawings do not exist yet, we bring in the technician who draws it and the structural engineer who calculates it, we manage the planning route, we notify Colchester City Council Building Control, and we run the trades as main contractor. We carry £2m public liability cover and everything is built to current Building Regulations. We notify Building Control, arrange the inspections and see the job through to the point where they issue your completion certificate.
We are based in West Bergholt and work across Colchester and roughly thirty miles around. The survey and the quotation cost nothing and there is no pressure afterwards.
Last reviewed August 2026. Planning rules and Building Regulations change, and your property may differ, so treat this as a guide rather than advice on your specific job.