Rear Extensions

What a rear extension actually is

New floor area built off the back wall of the house, out into the garden, under its own roof.

A rear extension projects backwards from the rear elevation. It can be single storey, two storey, or single storey over part of the footprint with two storeys over the rest. It has its own foundations, its own walls built to current insulation standards, its own roof, and a structural opening cut through the old back wall so the new space and the old space read as one room rather than two.

That last part is what separates a rear extension from a conservatory or a garden room. A conservatory is exempt from most Building Regulations on its size and its glazing, and the energy requirements stay off it only while external quality doors keep it outside the thermal envelope of the house. A garden room is a detached outbuilding under a different set of permitted development rules. A rear extension becomes part of the house, permanently, and is assessed as such.

The difference from a side extension is direction and therefore rules. A side extension goes out into a driveway or a side passage, and permitted development treats it far more strictly: single storey only, no wider than half the width of the original house, and it usually loses its permitted status altogether in a conservation area. A side return fills the narrow alley beside a Victorian terrace’s rear outrigger. A wrap around does both at once, going back and round the side in a single L.

The construction of all four is broadly similar. What changes is the planning position, the access, and where the daylight is going to come from.

The Larger Home Extension prior approval route

A way of going deeper without a planning application, but it is not a formality and it is not instant.

Where the standard depth is not enough, there is a second permitted development route that allows a deeper single storey rear extension. It is not a planning application. It is a notification to the council, and it comes with a neighbour consultation process.

You submit a description of the proposal with a plan and an elevation before any work starts. The council writes to the adjoining owners. They have a period in which to object. If nobody objects, the council issues its decision and you build. If a neighbour does object, the council then considers the impact on the amenity of adjoining properties, and only that: not design, not overlooking generally, not whether they like it.

The whole process runs to 42 days from a valid submission. That is a legal maximum for the council to determine it, not a target, and the clock does not start until the submission is valid.

Two practical points. First, the deeper extension still has to meet every other permitted development condition on height, eaves and materials. Second, the neighbour relationship matters more here than anywhere else in the process. We would always rather you had spoken to the people either side before a letter from the council lands on their mat, because a neighbour who has already seen the drawing and been told what the noisy weeks look like is a very different neighbour from one who finds out by post.

Foundations, and why the ground sets your price

The largest cost variable on the entire job is invisible until somebody digs a trial hole.

Most of the ground in and around Colchester is shrinkable clay, with pockets of gravel nearer the Colne valley. Clay takes on water in winter and swells, dries out in summer and shrinks. Close to the surface that movement is seasonal and it is more than enough to move a building.

A foundation founded inside that zone of movement will rise and fall with it. The existing house is founded deeper, or has simply settled and stopped moving decades ago, so it stays put. The extension does not, and the two part company at the junction. It shows as a diagonal crack at the corner where new meets old, usually in the second or third summer, and putting it right costs more than doing it correctly ever would have.

The answer is to found below the zone of seasonal movement. On an open site with no significant vegetation nearby that is commonly around 900mm, though the actual figure comes from the plasticity of the clay on your plot rather than from a rule of thumb. Where the depth needed becomes uneconomic to dig as a trench fill, the answer moves to a piled foundation with a reinforced ground beam and a void former beneath it, so the clay has somewhere to swell into without lifting the building.

The structural opening

Taking out the back wall is the structural event of the whole job, and it is the part that cannot be tidied up afterwards.

There is little value in building twenty square metres of beautiful new space and connecting it to the house with a doorway. The gain comes from removing most or all of the original rear wall so the kitchen, dining and new area behave as one room. That wall is carrying first floor joists, frequently the roof load above, and sometimes a chimney breast. All of it has to be picked up.

It goes into a steel beam, sized by an independent structural engineer from the actual loads on your house rather than from what was used next door. The beam bears onto padstones at each end. The padstone spreads a very concentrated point load into masonry that was never built to receive it, and the engineer checks whether that masonry can take it. Where it cannot, the load is carried down to foundation level through a pier or onto a new pad, which is why the opening sometimes affects the groundworks stage weeks before the steel appears.

Everybody wants a wide opening and a slim beam. Beam depth is a function of span and load and there is no arguing with it. On a wide span the options are a deeper beam concealed within the floor zone above, a pair of beams, or a goalpost frame with posts. What is not an option is asking for a smaller steel and hoping.

Temporary support while the wall is out is its own design and its own risk. Needles through the wall at calculated centres, propped above and below, loaded properly before a single brick is cut. This is the stage where cutting corners does not produce a crack later. It produces a collapse on the day.

Rooflights, and where light actually comes from

Glass at the back of a deep extension lights the back of it. The middle needs light from above.

A horizontal rooflight receives light from the whole sky rather than from one direction, and delivers substantially more usable daylight than a vertical window of the same area. It also delivers it in a much more even way across the depth of the room, which is the thing people notice without being able to name.

Position matters more than size. Rooflights placed near the rear wall of the extension, where the bifolds already flood the space with light, add very little. Set them back over the middle of the plan, and better still over the line of the old external wall, and they light the part of the house that was dark before you started.

There is a real cost difference between rooflight types and it is worth understanding what you are buying. A simple double glazed unit in a proprietary kerb upstand is the economical answer. A flush glazed frameless unit looks considerably better from inside and outside and costs multiples of it. A large single pane starts to need mechanical handling on site, which affects access as well as price.

Two practical notes. Any rooflight facing anywhere near south wants a solar control coating, or the room under it becomes uninhabitable on a July afternoon. And a rooflight over a kitchen island needs its position agreed against the island position, the ceiling structure and the lighting layout, before the roof structure is built rather than after.

The Party Wall etc. Act 1996

Not planning, not Building Regulations. A separate legal duty with fixed notice periods that cannot be shortened except where the adjoining owner consents in writing, which is the only thing that shortens them.

On a semi or a terrace, a rear extension almost always engages the Act. Three sections matter.

Notice is served by you as building owner, in writing, on every adjoining owner including freeholders and long leaseholders. The neighbour can consent in writing, which ends the matter, or dissent, at which point surveyors are appointed and an award is drawn up setting out how the work will be done. A neighbour who does not reply is deemed to have dissented after fourteen days.

Two things people get wrong. The periods run from service, so serving late does not compress them, it just delays the start. And a dissent is not hostility. It is often a neighbour being sensibly careful, and an award with a schedule of condition protects you as much as it protects them, because it records exactly what their plaster looked like before you started.

We flag party wall requirements at survey and build the periods into the programme. The surveyor is a third party and their fee sits outside our price, which is why it appears in the exclusions further down this page.

Full plans or building notice

Two routes into Building Control, and on a rear extension only one of them is sensible.

A building notice tells the council you are starting. There are no drawings to check up front and no formal approval, just inspection as the work proceeds. It suits small, simple, entirely conventional work where nothing is going to be argued about.

A full plans submission puts the drawings and the structural calculations in for assessment before anyone breaks ground, and produces a decision notice. It takes longer at the front end and it removes almost all of the risk of an expensive disagreement halfway up a wall. Any conditions or queries arrive when they cost a phone call rather than when they cost a rebuild.

On anything that involves removing a load bearing wall, which is every worthwhile rear extension, we use full plans. It also gives the client something to hold: an approved set showing exactly what was agreed, which matters if there is ever a question about what was built.

We notify Colchester City Council Building Control, we book the inspections at the stages that need them (foundation excavation before concrete, damp proof course, drains before covering, insulation, and completion) and we are on site for them. At the end you receive a completion certificate. Keep it with the deeds and with the Lawful Development Certificate. Those two pieces of paper together answer every question a future buyer’s solicitor is going to ask.

What a rear extension costs

Ranges with their basis stated, because a single figure from someone who has not seen your ground is worthless.

Costs move with materials, labour and specification, so treat everything here 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 rear extension on reasonable ground with decent access runs £2,200 to £3,000 per square metre for the shell and a standard finish. A two storey rear extension is typically lower per square metre, because you get a second floor of area from one set of foundations and one roof, though the total is obviously higher. Difficult access, deep foundations, wide structural openings, extensive glazing and a high specification all push above the range, and there is no upper limit on the last of those.

The range normally includes groundworks and foundations to the specified depth, drainage connections, the structure and the steel, the roof, windows and external doors, plastering, first and second fix carpentry, electrics and plumbing to a standard specification, and making good where new work meets 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 where a neighbour dissents, service diversions, landscaping, and anything genuinely unforeseeable found once the ground is open.

Colchester housing stock, and what suits what

The right rear extension for a New Town terrace is not the right one for a Highwoods estate house.

Victorian and Edwardian terraces in New Town and around Military Road generally have a rear outrigger, a party wall on both sides, and no vehicle access to the back. The most effective move is usually a side return infill combined with a modest rear projection, roofed with rooflights along the boundary. Party wall notices are a certainty rather than a possibility, and access costs are real, because everything comes through the hall on boards.

1930s semis in Lexden, Prettygate and along the older suburban roads are the classic rear extension house: cavity walls, a decent depth of garden, and frequently a side passage that gives access during the build. They often already carry a small post war or 1970s addition across part of the back, which has consumed depth allowance and is sometimes best removed and rebuilt as one.

Post war housing in Greenstead, Monkwick and parts of Shrub End sits on generous plots, which is good, but a proportion of it is non traditional construction. Some of these were not built with conventional cavity masonry, and the wall you want to open up may not behave like brickwork. That gets established at survey, before any structural design is done, because it changes the whole approach.

Modern estate housing in Stanway, Myland, Highwoods and Braiswick tends to have shallow gardens, close boundaries and, very often, removed permitted development rights or a developer covenant. The extension itself is usually simple. The paperwork in front of it is not.

What we get asked to put right

The same short list of failures, on other people’s work, over and over.

Foundations too shallow for the clay, cracking away from the house at the junction within two summers. Cold deck flat roofs with blocked or absent ventilation, condensing on the underside of the deck and rotting the joist ends. A steel bearing straight onto brickwork with no padstone, crushing the masonry beneath it. Missing cavity trays where the new roof abuts the existing wall, so water tracks through and appears on the bedroom wall above.

Then the ones that are not defects in the building at all. A manhole built over with no access, which is fine until the drain blocks. Rooflights placed where the light was already good. Bifolds facing south west with no solar control, in a room that cannot be used between two and six in the afternoon in summer. And the dark middle room, which is not bad workmanship, just a design that nobody thought about hard enough at the only point when thinking was free.

None of this is exotic and none of it is bad luck. Every item on that list is a decision that was made cheaply at the time and is expensive to reverse. When we are asked to price putting one right, the cost is nearly always higher than the saving that caused it, and that is before the disruption of doing the same work twice.