Can I knock down an old extension and rebuild it in London?

How far can the replacement go under permitted development?

For a single-storey rear extension, paragraph A.1(f) says it is not permitted development if it would extend beyond the rear wall of the original dwellinghouse by more than 4 metres in the case of a detached dwellinghouse, or 3 metres in the case of any other dwellinghouse, or rise above 4 m. Government guidance says the depth is measured from the base of the rear wall of the original house to the outer edge of the new wall.

That matters most on Victorian and Edwardian terraces, where the back addition (the narrow two-storey outrigger) is usually original and the single-storey kitchen behind it often is not. Knock down that later kitchen and the 3 m is counted from the original rear walls, which can be stepped. Our guides to extending a terraced house and extending a semi-detached house set out the depth, side and two-storey limits for each house type.

A second test catches rebuilds on small plots. Paragraph A.1(b) rules out an extension if buildings other than the original house would cover more than 50% of the total area of the curtilage, not counting the original house's own footprint. Sheds, garden rooms and the new extension all go into that sum. Where a rebuild is close to the limits, a lawful development certificate puts the council's confirmation on record before you build; the lawful development certificate cost guide explains what that involves.

Is taking the old extension down itself 'development'?

Section 55(1A) of the Town and Country Planning Act 1990 says building operations include demolition of buildings, so demolition can need planning permission on its own. The Secretary of State's 2021 demolition direction takes some demolition out of that: a building of not more than 50 cubic metres, measured externally, is not development. But the direction also says that 'building' there does not include part of a building, so a rear extension joined to a house cannot rely on the 50 m³ exclusion.

Whether pulling down an attached extension is 'demolition of a building' at all, or simply part of the works of altering the house, is a legal question that depends on how much comes down. The permitted development right to demolish buildings in Part 11, Class B of the 2015 Order is written for demolishing a building, with a prior approval step on method and restoration. In practice, when an old extension comes down to make way for a new one, the planning question that matters is whether the new extension is permitted development or has permission. Where planning permission is granted for the redevelopment, Part 11 counts demolition on that land as excluded demolition, which takes it out of the prior approval step, though not out of the conservation area rule below. Where the extent of demolition is unclear, we ask the council before work starts rather than assume.

What changes in a conservation area?

Conservation areas are article 2(3) land, and that removes several routes at once. Paragraph A.2 of Class A rules out any extension beyond a side wall of the original house and any rear extension of more than a single storey, and the 6 m prior approval route for single-storey rear extensions does not apply there either.

The demolition itself also needs care. Section 196D of the 1990 Act makes it an offence to carry out relevant demolition without the required planning permission, and relevant demolition means demolishing a building in a conservation area in England. Part 11 permitted development does not cover relevant demolition. Removing a small later lean-to from a house is a very different thing from clearing most of a building, but the line between the two is a matter of fact for the council. For any rebuild in a conservation area, we put the demolition and the new extension in one householder application, so the drawings show exactly what comes down and what goes up, and the council grants permission for both.

Building regulations, the demolition notice and the structure

Building regulations apply to the new extension whichever planning route it takes. Regulation 3(1)(a) of the Building Regulations 2010 lists the erection or extension of a building as building work, so the rebuild needs approval from building control, with foundations, insulation, drainage and fire safety to today's standards rather than those of the original. Our building regulations drawings give the builder and the inspector that detail.

Demolition has its own notice. Section 80 of the Building Act 1984 covers demolition of the whole or part of a building, apart from an internal part of an occupied building and very small structures, and the work cannot start until the council has replied or six weeks from the giving of the notice have passed. An old extension often supports more than it seems: a beam over the opening into the house, a chimney breast removed when it was built, or a roof tied into the main wall. The structural calculations for the new opening and foundations cost £745 with us, and they go to building control with the drawings.

The neighbours and the Party Wall Act

Old extensions were often built on shallow footings, and a replacement built to current standards usually goes deeper. Section 6 of the Party Wall etc. Act 1996 applies when you excavate within a distance of three metres measured horizontally from a neighbour's building and go below the bottom of its foundations. Notice must be served at least one month before the excavation starts.

On a terrace or semi, rebuilding against the shared wall, or cutting into it for a new beam, brings in the party structure rules as well. Those notices are separate from planning and building control, and a neighbour has the right to appoint a surveyor. Talking to the neighbours with the drawings in hand before the old extension comes down saves time once the builder is booked.