House Extensions in South London

Four shapes cover nearly every extension on a South London terrace or semi, and the same parts sit under all of them.

Many single-storey rear extensions need no planning application. Householder permitted development allows one on a house — flats and maisonettes have no such rights — within fixed limits. As the rules stand, it can go up to 3 m deep on an attached house, meaning a terrace or a semi, and 4 m on a detached one. It can be no more than 4 m high, with the eaves no higher than 3 m within 2 m of a boundary, in materials similar in appearance to the house.

Outside a conservation area, the larger home extension route stretches that to 6 m and 8 m, but it needs prior approval from the council and your neighbours are consulted first. A side extension can be single storey, up to 4 m high and no wider than half the original house. A two-storey rear extension can go up to 3 m deep and must stay at least 7 m from the rear boundary. Extensions and outbuildings together may not cover more than half the garden land around the original house.

Then the exceptions, and in South London they are common. A conservation area removes permitted development for side extensions, for rear extensions of more than one storey and for cladding. An Article 4 direction can remove more. A listed building needs listed building consent. A wrap-around is judged against the rear and side limits at once, so it is the shape most likely to need a full application. A lawful development certificate from the council settles in writing whether a design is permitted development. Check every figure here with the council or the Planning Portal first.

Planning decides whether you may build it. Building Regulations decide how it is built, and an extension needs Building Regulations approval whether or not it needed planning. Building control inspects the work at stages and issues a completion certificate at the end — the paper a buyer's solicitor asks for when you sell.

The Party Wall etc. Act 1996 catches most extensions on a terrace or semi. As the Act stands, notice goes to the neighbour one month ahead for a new wall on the boundary line, two months ahead for work to the party wall itself, such as cutting in a steel, and one month ahead for digging within 3 m of their building to below their foundations, or within 6 m in some cases. They can consent in writing, or a surveyor settles an award.

Behind many terraces a shared sewer runs under the gardens. As the rules stand, building over a public sewer, or within 3 m of one, needs a build-over agreement from the water company — Thames Water across South London. It can change where the foundations go, so find the sewer before the design is fixed.

Then the ground. London clay shrinks in dry summers and swells in wet winters, and trees pull water out of it, a neighbour's as much as your own. Both usually mean deeper foundations than people expect, and building control sets the depth once the trench is open.

An extension is worth building because it opens into the house, and that means taking out part of the back wall. That wall holds up everything above it: the brickwork, often a floor, sometimes part of the roof. The opening is spanned with a steel beam sized by a structural engineer's calculation, bearing on masonry that can take the load down to a foundation. A chimney breast in the back room is the other common casualty, and removing one needs the same calculation, support and approval.

While the wall comes out, everything above it sits on temporary support, and building control will want to see the steel before it is boxed in. Where the steel bears into the party wall with next door, that is the two-month notice.

The order matters. Where the design allows, the new shell is closed in before the old wall comes out; where the steel has to go in first, the opening should be sheeted and boarded. Ask any builder when they plan to knock through, and what keeps the rain out in between.

Most single-storey extension roofs are flat, covered in EPDM rubber or GRP fibreglass over a warm roof, where the insulation sits above the deck so the timber stays warm and dry. A pitched lean-to in slate or tile suits some houses, but it needs enough height against the back wall to reach the pitch the covering demands. Roof lights sit on raised kerbs, so the covering can be dressed up the kerb and under the frame.

Where the new roof meets the old wall, the covering has to turn up the wall and be lapped by a lead flashing let into a mortar joint above it. On a cavity wall, a cavity tray over that line stops water in the cavity running down into the new room. On the back of a Victorian terrace, a first-floor window sill can sit just above the new roof and leave no height for a proper upstand. That has to be designed before the roof level is fixed, not solved with mastic on the last day.

A side return adds the hardest detail of all: the gutter between the new roof and the neighbour's wall, or between your extension and theirs. Water from one roof, sometimes two, runs into a channel nobody sees from the garden. When it fails, the water comes in along the party wall.

The roof can also decide whether the design is permitted development. A warm roof adds depth at the edge, and as the rules stand the eaves cannot go above 3 m within 2 m of a boundary, so the deck, the insulation and the fall belong on the drawings, not squeezed in once the walls are up.

Paperwork first: drawings, calculations, planning or prior approval where it applies, the Building Regulations application, party wall notices and any build-over agreement. The parts that affect the foundations have to be settled before anyone digs.

Then the ground: trenches dug and inspected, concrete poured, drains laid, walls brought up and the floor slab cast. The shell follows — walls, any steelwork the roof depends on, the roof structure, covering and flashings, windows and doors. Then the knock-through, first fix for the electrics and plumbing, insulation, plasterboard and plaster, second fix and decoration. Building control's completion certificate comes at the end, with the electrical and gas certificates wherever that work was done.

A fixed price in writing is only fixed on what it lists. These are the questions that find the gaps.