Infill Extensions

What an infill extension actually is

Building into a space the house already encloses on two or three sides, instead of out into the garden.

Look at the back of most older Colchester houses from above and the footprint is not a rectangle. There is a rear wing or outrigger sticking out, and beside it a gap open to the sky but walled on two sides by your own house. On newer houses the same shape is man made: somebody extended the back twenty years ago, stopped short of the corner, and left a courtyard nobody uses.

The word describes the shape of the site, not a planning category, and there is no relaxation because it is small. What is different is the ratio of old wall to new floor. On a twenty square metre rear extension you might build fifteen metres of new external wall and connect back to the house across five. On a nine square metre infill you might build four metres of new wall and have twelve metres of existing masonry to seal against, tie into and roof over.

That is the whole page in one sentence. An infill is not a small extension. It is a large amount of junction with a small amount of floor attached, and every junction is between something new and something old that was never designed to receive it.

Permitted development, and the wall your depth is measured from

Often permitted, and often by less than people assume, because someone has already spent part of the allowance.

Most single storey rear infills fall under permitted development, meaning permission has already been granted in general terms and no application is needed. The rights carry conditions on depth, height, materials and position relative to the boundary.

Depth is measured from the original rear wall: as built, or as it stood in 1948 for anything older. Not the wall that is there now. On an infill that matters more than on any other job, because the thing which created your gap was usually a previous extension, and that extension has already used part of the depth the rules allow.

Height is capped, and there is a lower cap on eaves height within two metres of a boundary. An infill is almost always within two metres of one, so that lower cap is usually the one doing the work, and it often decides the roof design before anything else does.

These limits change from time to time and how they apply depends on your property. Treat this as an explanation of the system rather than confirmation of what you can build, and note the review date at the foot of this page.

Foundations, Essex clay, and the footing next door that is far shallower

You are digging deep, right alongside something shallow, in ground that moves.

The ground around Colchester is largely shrinkable clay. It swells as it takes water and shrinks as it dries, and near the surface that seasonal movement is significant. Found inside that zone and the extension moves while the house does not, so it cracks away at the junction, usually in the second or third summer. The answer is to found below the zone, typically around 900mm on an open site with no trees nearby.

Here is the part specific to infills. Older houses often sit on footings 450 to 600mm down, sometimes less. Your new foundation has to go deeper and runs immediately alongside, sometimes within a few hundred millimetres. Dig a continuous open trench beside a shallow footing in wet clay and you undermine the house you are meant to be extending.

So it is dug in short bays, each concreted before the next is opened, with the sequence on the drawing rather than decided by whoever is on the digger. Where the existing footing is very shallow or very poor, the new foundation is stepped or locally deepened, and occasionally the old one is underpinned in bays first. That is a real cost, and it is found by digging a trial hole rather than by looking at the house.

Trees change the numbers again. Species, mature height and distance set the depth, and an oak, poplar or willow within influencing distance can take you to two and a half metres or into piles with a suspended slab. Taking the tree out does not solve it: ground dried by roots for decades takes that water back and swells, and heave acts upwards and does more damage than settlement.

Roof junctions, valleys, and where infills leak

If an infill fails, it fails at the roof, and almost always at a junction with an existing wall.

An infill roof does not stand alone. It runs into the main roof, the outrigger roof, or both, and abuts existing walls on two or three sides. Every abutment is a place water can track in, and unlike a rear extension the failure often shows up inside the old house rather than the new room.

The critical detail is the cavity tray. Where the new roof meets an existing cavity wall, rain running down the outer leaf has to be caught above the abutment and thrown back out, or it crosses the cavity and appears on the inside face above the new ceiling. On an infill the tray usually steps up two different walls at two different levels and turns a corner. It is fiddly, it is buried within a day of being built, and it is the most common thing we are called to find in someone else’s work.

The valley is the other one. Where an infill roof meets a pitched outrigger roof, water off both slopes funnels into one narrow channel, often finishing over a boundary. Valleys concentrate water and they concentrate leaves. They want proper lead or a designed valley system, a decent gutter and enough fall.

Most infills end up flat roofed, because eaves height near a boundary is capped and a pitch would foul the first floor windows. On a bungalow only the height cap applies. Flat is fine, done properly: laid to a fall, finished in single ply or GRP, and built as a warm deck with the insulation above the structural deck. A cold deck relies on ventilating the void above the insulation, and on a small roof enclosed on three sides there is nowhere for that air to come from or go to. Cold decks on infills rot. We build warm decks.

What an infill extension costs

Ranges with their basis stated, because a small area makes the rate misleading.

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, an infill on reasonable ground with workable access typically runs £2,400 to £3,200 per square metre for the shell and a standard finish. That is above the rate for a plain rear extension, for the reasons already given: fixed costs do not shrink with the floor area, the junction count is high, and the access is usually poor.

That range normally includes groundworks and foundations to the specified depth, drainage alterations within the site, structure and any steel, the roof and its abutment details, rooflights, 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, and you should budget separately for: VAT, kitchen units and appliances, flooring, decoration, drawings and calculations, planning and Building Control fees, the Anglian Water application, party wall surveyor fees if a neighbour dissents, service diversions, and anything genuinely unforeseeable once the ground is open.

The variables that move the number most, roughly in order, are the drain position, the depth of the footings you are digging alongside, access, the amount of overhead glazing and whether the boundary wall is yours. On a six square metre job, any one of them shifts the total by more than the floor finish will.

Questions we get asked

Is an infill cheaper than just extending out the back?

In total, almost always, because there is far less of it. Per square metre, no, it is usually dearer. The better question is what each option does to the plan. If the problem is a narrow kitchen, an infill fixes it and a deeper rear extension does not, because it only makes the same narrow room longer. If you need another room, going out the back gives you more floor for your money. Plenty of jobs do both at once, which costs less than doing them five years apart.

Will I lose access to the back garden?

On a terrace or semi where the side alley is the only route from front to back, yes, permanently. Afterwards, bikes, bins, garden waste and any future building materials come through the house. Some plots keep a route by leaving a covered passage at the end, which costs floor area. There is no clever answer, only a decision made with your eyes open rather than discovered at first fix.

What if the earlier extension was never approved?

It happens more than you would think, and it is a problem to solve rather than a disaster. There are time limits after which the council can no longer take enforcement action against a breach of planning control, and those limits changed recently, so the position gets checked against current law rather than assumed. Missing Building Regulations approval is separate, usually dealt with by a regularisation application or an indemnity policy at sale. What we will not do is build off the back of something unlawful without telling you first.

The boundary wall is my neighbour’s. Does that stop me?

No, but it changes the detailing and it brings the Party Wall Act in as a certainty rather than a possibility. You cannot build off their wall, cut into it or hang your roof on it without the right notices and agreement. The usual answer is to build your own wall just inside your line, losing a little width, or to serve notice and work to an agreed detail. Either way the wall is surveyed first, because an old boundary wall on a shallow footing is not something to lean a roof against.