How much does a side-return or wraparound extension cost in London?

A side return is the narrow strip of yard beside the back addition of a Victorian or Edwardian terrace. Building over it turns a dark galley kitchen into one wide room. A wraparound does that and also extends past the back wall, in an L shape around the back addition, so it adds much more floor and costs much more.

The table gives the all-in total for each at three finishes on a typical terrace, with the floor area assumed. Each range is wide on purpose. The low end is a plain job on an easy site. The high end is a harder site with more to put right.

Take a two-storey mid-terrace house with a side return 6 m long and 1.8 m wide. That adds about 10.8 m² of floor, though the room you end up with is far bigger, because the wall between the old kitchen and the yard comes out. With a good-quality finish, the building work is £36,000 to £60,000. Add the fees and the contingency and the all-in total is £45,000 to £87,000, with a likely figure of about £66,000.

The wraparound priced here is 4 m deep and 6.5 m wide, about 26 m² of floor. With a good-quality finish its building work is £62,000 to £103,000 and its likely all-in figure about £109,000.

Measured on the building work alone, a side return with a good-quality finish works out at about £4,450 per m² of new floor, in a range of £3,300 to £5,600. A single-storey rear extension at the same finish is about £3,750 per m², and a wraparound about £3,200. The side return is small in area but dense in work.

A long steel. The side wall of the back addition comes out along its full length, so a long beam, often with steel posts at each end, carries the wall and floor above. It needs a structural engineer's calculation and sometimes a crane or a team to lift it in.

A party wall. The new extension usually sits on or against the boundary with the neighbour, so the foundations and the new wall are party wall work, with notices and sometimes surveyors.

Drains to move. Side returns are where the drains of a terrace often run, so manholes and pipes are rerouted, and a public sewer close by may need an agreement with Thames Water before building over it.

A lot of glass per square metre. Most side returns have a glazed roof or large rooflights to bring light into the middle of the house, and glass costs more than a solid roof. None of this shrinks with the floor area, so the rate per m² rises as the room gets narrower.

The Party Wall etc. Act 1996 says you must tell the neighbours before building on or at the boundary, digging foundations near their house, or cutting into a shared wall. That is a party wall notice: a short written notice, served before work starts, describing what you plan to do. A side return on a terrace usually triggers it for the neighbour on the side-return side, and often for the other neighbour too if the back wall work touches theirs.

Each neighbour can consent in writing, and then no surveyor is needed. If a neighbour dissents, or does not reply, a party wall surveyor draws up an award setting out how the work is done and records the state of their house first. The neighbour can share your surveyor or appoint their own, and you normally pay for both.

The totals carry a party wall line of £0 to £6,000 for up to two neighbours. It is zero when both consent in writing and reaches the top of the range when both dissent and appoint their own surveyors.

The building work covers the groundworks, the structure, the roof and glazing, and the first and second fix of electrics, plumbing, plaster and joinery, ready to decorate. The totals then add the fees for a side return: architect or designer £3,000 to £8,000; structural engineer £1,000 to £2,500; council prior approval fee £334 with the Planning Portal charge; building control £1,000 to £2,000; party wall surveyors £0 to £6,000; Thames Water build-over agreement £0 to £1,000. Together the fees come to £5,000 to £19,500. The build-over line is zero if no public sewer runs within reach. The council fees are in the box below.

On top of the fees is a contingency of 12% of the building work at a good-quality finish, about £4,500 to £7,500 for a side return. It is money to have available for what only shows once the floor is up, such as an old drain or a weak wall, not a cost you will certainly pay.

Outside the totals: a new kitchen, which most side returns are built around, bifold or sliding doors beyond the standard glazing, flooring through the rest of the ground floor, furniture and blinds, and any time living elsewhere while the work is done.

Build costs move with parking, access and labour. On the London baseline a side return with a good-quality finish is £45,000 to £87,000. In Barking and Dagenham, Bexley and Havering, the lowest-cost end of the scale, it is £42,000 to £82,000. In Kensington and Chelsea and Westminster, the highest, it is £52,000 to £99,000.

The borough changes the building work, not the statutory fees or the planning rules. Each borough's page gives the totals for every job there.

A side extension is normally permitted development, subject to the limits checked in the report: single storey, no more than 4 m high, and no wider than half the width of the original house. A wraparound has to pass that side test and the rear depth test together, so past the standard depth it needs prior approval, and beyond the larger limit a planning application. For the typical side return priced here the answer is: larger home extension, prior approval needed. A conservation area takes side extensions out of permitted development and an Article 4 direction can too, so check with the council; the full report checks the planning register for your street.

Side-return extension costs, the planning route and a timeline in each London borough.

Barking and Dagenham · Barnet · Bexley · Brent · Bromley · Camden · Croydon · Ealing · Enfield · Greenwich · Hackney · Hammersmith and Fulham · Haringey · Harrow · Havering · Hillingdon · Hounslow · Islington · Kensington and Chelsea · Kingston upon Thames · Lambeth · Lewisham · Merton · Newham · Redbridge · Richmond upon Thames · Southwark · Sutton · Tower Hamlets · Waltham Forest · Wandsworth · Westminster

Is a side return cheaper than a rear extension?

In total, usually yes, because it adds less floor. A side return of about 10.8 m² is £45,000 to £87,000 all in at a good-quality finish; a single-storey rear extension of about 17.5 m² is £61,000 to £114,000. Per square metre the side return costs more, about £4,450 against £3,750 for the building work, because the steel, the party wall and the glass do not shrink with the room.

How long does a side-return extension take?

About 23 to 41 weeks from first sketch to sign-off, of which 8 to 14 weeks are on site. Design, the council's decision, the party wall notices and the building regulations drawings take most of the rest, and the notices need time to run before work can start.

Do these side-return prices include VAT and fees?

Yes. Every figure includes VAT at 20%, and the all-in totals include the professional and council fees, £5,000 to £19,500 for a side return, and a contingency of 12% of the building work at a good-quality finish (10% at straightforward, 15% at high end). They are an estimate to plan with, not a quote.