Two Storey Extensions
Twice the floor area, a different structural problem, and almost always a full planning application.
A two storey extension is the best value floor area you can add to a house, because two floors come out of one set of foundations and one roof. It is also the point at which planning stops being a formality and the structure stops being simple.
What a two storey extension gives you
Roughly twice the space for well under twice the cost.
The economics are the reason people build them. The two most expensive elements of any extension are the groundworks below and the roof above. A two storey extension pays for those once and gets two floors out of them, which is why the cost per square metre is typically lower than a single storey extension of the same footprint, often by several hundred pounds a metre.
What that buys in practice is usually a kitchen and dining space downstairs and a bedroom, frequently with an en suite, upstairs. On a three bedroom semi it is the move that turns the house into a four bedroom family home without moving, and in the Colchester market that is a meaningful change in what the property is worth as well as what it is like to live in.
The trade offs are real and worth stating. A two storey extension takes longer, disrupts the whole house rather than just the ground floor, and almost always needs planning permission rather than falling under permitted development. It also affects your neighbours far more than a single storey does, which changes how the application is received.
Planning: expect an application
Two storey extensions do have permitted development rights. They are just narrow enough that most designs fall outside them.
Permitted development for a two storey rear extension exists, but the conditions are considerably tighter than for single storey work. The depth limit is shallower, there is a minimum distance the extension must keep from the rear boundary, and the roof pitch is required to match the existing house as far as practicable. Any part within two metres of a boundary is capped at a lower eaves height, and upper floor side windows must be obscure glazed and non opening below a set height.
Add those together on a typical Colchester plot and most two storey schemes exceed at least one of them, which means a full householder planning application. That is not a problem. It is a process with a statutory eight week determination period, and it is far better to plan for it than to contort the design trying to squeeze under a limit that will compromise the rooms.
What the council will actually weigh is the effect on neighbouring amenity: daylight and sunlight, overlooking, and whether the extension is overbearing when seen from next door’s garden or windows. A two storey mass close to a boundary is the classic objection, and it is the reason a design that steps in at first floor level, or sets the upper storey back, often sails through where a full height box does not.
Nobody can promise you a planning outcome, and anyone who does is telling you something they cannot know. What can be done is designing against the local plan policies from the outset, and dealing with the predictable objections inside the application rather than after a refusal notice.
If the property is in a conservation area, or listed, or covered by an Article 4 direction, the position changes again. Estate housing built from the 1980s onwards frequently had permitted development rights removed by a condition on the original estate permission, which almost nobody checks. We look at the planning history for the address before anything is drawn.
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.
The party wall position
On a semi or a terrace this is not optional, and the notice periods are fixed in law.
A two storey extension on a semi detached house will almost certainly engage the Party Wall etc. Act 1996, usually on more than one count. Cutting into the shared wall to tie the new structure in falls under section 2. Building up to the boundary line falls under section 1. Excavating within three metres of the neighbour’s structure to a depth below their foundations falls under section 6, and on Colchester’s shallow interwar footings that last one is triggered far more often than people expect, because the new foundation is deep and the neighbour’s is not. A detached house is not exempt from that last one.
Notice periods are one month or two months depending on the section, and they cannot be shortened by agreement being reached quickly or by anyone being in a hurry. The one thing that does shorten them is the adjoining owner consenting in writing. If your neighbour dissents, surveyors are appointed and an award is drawn up, and you pay for it as the building owner.
None of this is a dispute and it is not a sign that anything has gone wrong. It is a routine part of building on an attached house. It only becomes a problem when it is left until the week before you wanted to break ground, because a statutory period is a statutory period.
The practical advice that costs nothing and works: tell your neighbours yourself, with the drawings in your hand, before a letter arrives from the council. An objection is far more often a reaction to being surprised than a considered planning argument, and once it is on the public file it stays there whether or not they later change their mind.
Structure: it is a different job upstairs
The load path has to reach the ground, and on an existing house that is rarely a straight line.
On a single storey extension the structural question is usually one beam over one opening. On a two storey extension you are adding a floor, a roof and everything on them to a building that was designed to carry none of it.
The first floor structure spans somewhere, and wherever it lands the load has to continue down through the ground floor walls and into foundations capable of taking it. Where the new first floor sits over a new ground floor opening, the beam over that opening is carrying the wall above as well as the floor, and it gets substantially bigger than the equivalent beam on a single storey job.
Tying the new structure to the old is its own piece of design. Wall starter systems, or toothing the new brickwork into the existing, with the engineer specifying which and where. The junction has to transfer load and accommodate the fact that the two structures will move very slightly differently for the first year or two.
Then there is the existing house. A two storey extension frequently means removing part of a load bearing wall at ground floor and at first floor, which means temporary support on two levels at once. That is where the risk on this type of job actually sits, and it is why the propping and needling arrangement is designed rather than improvised.
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.
Foundations under a two storey load
More load, same clay, and usually a deeper dig than the single storey version of the same extension.
Most of the ground around Colchester is shrinkable clay, which swells when wet and shrinks when it dries. Founding inside the zone where that happens means the extension moves relative to the house it is attached to, and cracks away from it, usually in the second or third summer.
Two storey work does not change the depth needed to escape seasonal movement, which is set by the soil and any trees nearby. It changes the width and reinforcement of what sits at that depth, because there is roughly twice the load coming down. On a plot with a mature oak, lime or poplar within influencing distance, the sensible engineering answer frequently moves to a piled foundation with a suspended reinforced ground beam and a compressible void former beneath it.
That sounds expensive and it is, up front. It is considerably cheaper than underpinning a cracked two storey extension four years later, and it is the difference that explains most suspiciously low quotes on this type of work.
Ask any builder pricing a two storey extension what foundation depth and type they have allowed for, and what the mechanism is if the ground requires more. A quote with no stated assumption about the foundations has not priced the job.
The roof, and why matching matters more here
A single storey roof is barely visible from the street. A two storey roof is part of the house.
Permitted development requires the roof pitch to match the existing house as far as practicable, and even where you are making a full application the council will look hard at it, because a two storey extension changes the silhouette of the building in a way a single storey one does not.
The usual options are a gable end, a hipped roof to match an existing hip, or a flat roof set behind a parapet. On the interwar semis common across Lexden, Prettygate and Shrub End, the existing roof is hipped, and continuing that hip over the extension is what makes the addition read as part of the house rather than bolted onto it.
Valley junctions are where two storey roofs leak. Where the new roof meets the existing, the detail carries a great deal of water and it has to be lead or a properly specified alternative, dressed correctly, with the cavity tray above it. This is invisible in the finished building and it is the single most common source of a call four winters later.
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
Building Regulations at two storeys
Everything a single storey extension needs, plus fire.
Part A for structure, C for damp and cavity trays, F for ventilation, H for drainage, K for stairs and guarding, L for insulation and glazed area, all apply as they would on any extension.
Part B is where two storey work differs. Adding habitable rooms at first floor level engages the escape provisions: habitable rooms need an escape window of adequate size and position, and where the extension changes the escape route from the existing first floor, that route may need protecting. On most two storey rear extensions this is straightforward, but it is a design input rather than a detail you resolve on site.
Part L also bites harder, simply because there is more external wall and more roof. The insulation build up in a two storey extension typically pushes the wall thickness beyond what people expect, and that has to be in the drawings from the start because it affects the internal dimensions of every room.
We use a full plans submission on this type of work rather than a building notice. The drawings and the engineer’s calculations are checked before anyone starts, and you receive a decision notice, which removes almost all the risk of an expensive disagreement partway up a wall.
What it does to the rest of the house
The part people plan for least.
A single storey rear extension is built almost entirely outside the existing wall, so the house stays intact behind it for most of the programme. A two storey extension is not like that. At some point the first floor is opened up, which means the bedrooms are affected, and for a period the house has scaffolding around it and a hole in the back wall on two levels.
Most families stay in the house throughout, and it is manageable, but it is worth going in with an accurate picture. The disruptive phase is longer than on single storey work and it reaches the whole building rather than just the ground floor.
There is also the question of what the new upstairs room does to the landing. Adding a bedroom off an existing landing is simple. Adding one that needs a new section of landing, or that turns a two bedroom landing into a four bedroom one, sometimes means moving a wall or reconfiguring the stairs, and that is a bigger job than the extension itself in some houses.
The other consideration is daylight to the existing rooms. Building two storeys across the back of a house takes light from the ground floor middle room and from any first floor window that looked out that way. On a terrace or a narrow semi that is the most common regret, and it is entirely designable at drawing stage with a set back, a light well, or choosing between rooflights, lanterns and a glazed roof.
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.
Building over an existing garage
A common two storey brief, and the one that most often turns out not to be possible as drawn.
Adding a bedroom above an attached garage looks like the cheapest two storey extension available, because the walls and the slab already exist. Frequently it is. Occasionally it is not, and the difference is entirely in what is under the ground.
Attached garages on interwar and post-war housing were built to carry a car and a lightweight roof. The foundations are often shallow, sometimes very shallow, and the walls may be single skin rather than cavity. None of that is adequate to carry a habitable floor, a roof and everything on them.
The assessment is a trial hole: dig down beside the existing foundation, see what is actually there, and let the engineer decide. There are three usual outcomes. The existing foundation is adequate and you build straight off it. It is inadequate but can be underpinned, which adds cost but keeps the structure. Or it is not viable and the garage comes down and is rebuilt from new foundations, at which point the economics change completely and it is worth reconsidering the whole scheme.
Nobody can tell you which of those applies from the pavement. Anyone who quotes a build-over-garage job without a trial hole is pricing a guess, and the difference between the three outcomes is easily twenty thousand pounds.
The single skin question matters too. A garage wall built in one brick skin has no cavity, so it cannot meet Part L as a habitable room wall and it cannot be made to without either an internal insulated lining that eats the room width, or rebuilding the wall. Both are legitimate answers, and both need deciding before you have a layout you are attached to.
Insulation, and a wall thicker than you expect
Part L on a two storey extension has consequences for the internal dimensions of every room.
Modern insulation standards mean an external wall build up is considerably thicker than the wall on the house you are attaching to. On a two storey extension that thickness applies over two floors and around a larger perimeter, and it eats internal space in a way that surprises people who measured the footprint on the drawing and assumed the rooms would match.
This has to be in the drawings from the start, not resolved on site. A bedroom that works at 3.2 metres wide and does not at 3.0 metres is a decision made by the wall build up, and it is better made deliberately.
Thermal bridging is the detail that decides whether the extension performs the way the calculation says. Cold bridges at the wall to floor junction, at the first floor junction, around every opening and at the eaves are where heat leaves and where condensation forms on the inside face in winter. Insulated cavity closers around every opening, continuity at the floor junction, and insulation carried properly at the eaves. None of it is visible and all of it is the difference between a warm extension and a cold one.
The other consequence people miss: adding this much heated floor area means the existing boiler is now serving a substantially larger load. On an older system that is worth checking rather than assuming, and it is cheaper to upgrade while the walls are open than afterwards.
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
What a two storey extension costs
Lower per square metre than single storey, higher in total, and the same variables drive it.
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 two storey extension on reasonable ground with decent access typically runs £1,900 to £2,600 per square metre of total floor area for the shell and a standard finish. On a 30 square metre footprint that is 60 square metres of floor across two levels, which puts a typical project somewhere in the region of £114,000 to £156,000 before the exclusions below.
That range normally includes groundworks and foundations to the specified depth, drainage, structure including the steels and padstones, external walls and insulation, roof and rainwater goods, windows and external doors, plastering, first and second fix carpentry, electrics and plumbing to a standard specification, and making good where new meets old.
It normally excludes: VAT, kitchen units and appliances, bathroom and en suite sanitaryware, flooring, decoration, professional fees for drawings and structural calculations, planning and Building Control fees, party wall surveyor fees where a neighbour dissents, service diversions, and anything genuinely unforeseeable found once the ground is open.
The variables that move the number most, in order: foundation depth and type, access, the structural spans at both levels, the roof form, and glazing specification. A two storey extension on a house with no side access, where every barrow of muck and every pack of blocks goes through the hallway, costs meaningfully more than the identical extension on a house with a side gate.
How long it takes
Longer at both ends than a single storey job.
The front end is the part people underestimate. A householder planning application carries a statutory eight week determination period, and that clock only starts once the application has been validated, which can add a week or two if anything is missing. Building Regulations full plans approval runs alongside. Party wall notices run one or two months and cannot be compressed except where the adjoining owner consents in writing, which is the only thing that shortens them.
Those overlap and we run them in parallel wherever possible, but a realistic front end for a two storey extension is three months, and longer if the first application is refused and the scheme is amended and resubmitted.
On site, a two storey extension is commonly sixteen to twenty six weeks from breaking ground to completion, depending on size, foundation type and finish level. Weather affects groundworks and roofing, and a wet winter moves a programme in ways nobody controls.
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.
Is a two storey extension the right move?
Sometimes the honest answer is a loft conversion.
If what you actually need is one more bedroom, a loft conversion is frequently cheaper, faster, less disruptive to the ground floor and does not need planning permission in most cases. It also does not take any garden. On a hipped interwar semi, a hip to gable conversion produces a genuine double with an en suite for considerably less than a two storey extension.
A two storey extension wins where you need space on both floors, where the loft genuinely does not work because of head height or roof structure, or where the ground floor needs to grow anyway and adding the upper storey is comparatively cheap once the foundations are going in. On a bungalow the whole first floor is the extension.
It also wins where the ground floor layout is the real problem. A loft conversion does nothing about a small dark kitchen.
We will tell you honestly which of those your house is, including when the answer is the smaller job. There is no version of this where selling you something that does not solve your problem works out well for anybody.
Questions we get asked
Will it definitely need planning permission?
Usually, yes. Two storey permitted development rights exist but the depth limit, the rear boundary distance and the roof pitch requirement are narrow enough that most schemes fall outside at least one. We check the position on your address, including any condition on the original estate permission, before designing anything.
Can we stay in the house?
Almost always. It is more disruptive than a single storey extension because the first floor is involved and the whole building is scaffolded, but the house remains habitable. The concentrated disruption is around opening up the two levels, and we plan that with you rather than announcing it.
Do I need to tell my neighbour?
Under the Party Wall Act, on a semi or terrace, you serve formal notice as the building owner. That is a legal requirement rather than a courtesy, and the notice periods are set in law. Separately, telling them informally with the drawings before the council writes to them is the single most effective thing you can do for the application.
Will it add value?
Turning a three bedroom house into a four bedroom house generally does, and in the Colchester market a well built two storey extension with a proper kitchen and an extra double bedroom is usually a sound investment. We are builders rather than valuers, so if the numbers matter to your decision, ask a local agent what the ceiling price is for your street before you commit.
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.
Neighbours, and how objections actually work
A two storey extension is the type most likely to attract an objection, and most objections carry less weight than people fear.
Neighbours are consulted on a planning application and can comment. They cannot veto it. An objection only counts where it raises a genuine planning consideration, and a surprising amount of what people write about is not one.
Material considerations, which the officer must weigh: loss of daylight or sunlight, overlooking and loss of privacy, an overbearing or enclosing effect, the character of the area, highway safety and parking. On a two storey extension the first three are the ones that bite, and they are the reason a design that steps the upper floor back from the boundary, or angles a window away, frequently succeeds where a full height flush box does not.
Not material considerations, whatever anybody feels: loss of a view, effect on property value, disruption during construction, the identity of the applicant, or a private boundary dispute. A boundary disagreement is a legal matter between the two of you and the council has no role in it.
The practical approach that works costs nothing. Show your neighbours the drawings before the council writes to them. Explain what it looks like from their side, and where their light goes. Most objections are a reaction to being surprised by a formal letter rather than a considered planning argument, and once one is submitted it sits on the public file whether or not the person later changes their mind.
Where an objection is genuine, it is usually better to amend the scheme than to fight it. A refusal notice tells you precisely what the problem was, and amending and resubmitting is almost always faster than an appeal to the Planning Inspectorate, which takes months.
How we do it
One contract, one contact, and the paperwork is our job.
Our team has 25+ years in the trade and comes out to survey the property. We look at the ground, the trees, the access, the existing structure and footings where we can see them, the drain runs and the planning position.
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 produces the drawings and the structural engineer who does the calculations for both levels, we manage the planning application, we handle the party wall notices, we notify Colchester City Council Building Control and we deal with the inspections through to your completion certificate. We work as main contractor, which means the trades answer to us and we answer to you.
We carry £2m public liability cover and everything is built to current Building Regulations. 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.