Wraparound and Double Storey Extensions in London: Costs, Rules and Build Times

There’s a point where a small extension stops being worth it.

You work out that three metres off the back gives you a decent kitchen, but the dining room is still dark and the box room upstairs is still a box room. So the question changes. Instead of asking how far out you can go, you start asking whether to go round the side as well — or up.

That’s where wraparound and double storey extensions come in. Both are a bigger commitment than a standard rear extension, and both bring problems the smaller job doesn’t have. Worth knowing what those are before you fall in love with a drawing.

What a wraparound extension is

A wraparound is a side return and a rear extension built as one. It fills the narrow alley down the side of the house and continues across the back, so the new floor area comes round the original building in an L.

It’s the most space you can add at ground level without taking most of the garden. On a Victorian terrace it also solves a problem the two separate extensions don’t quite fix on their own: the awkward corner where the old outrigger meets the back wall. Build both at once and that corner disappears.

The catch is what’s already in that side alley. Drains, usually. A soil stack, an inspection chamber, sometimes a shared drain running under where you want your foundations. That’s not a reason to abandon the idea, but it changes the sequence and the cost, and it needs to be found at survey stage rather than in week three.

What a double storey extension is

The same footprint, twice. Usually a rear extension with a bedroom or bathroom over it.

The appeal is obvious. You pay once for scaffolding, once for the roof, once for the disruption, and you get roughly double the floor area. Per square metre it’s the cheapest way to add space to a London house. If you know you’ll want the extra bedroom eventually, building both floors at once costs far less than coming back in five years.

What people underestimate is that the ground floor extension and the first floor over it are not the same job twice. The load path changes. Everything below has to carry what’s above, which usually means deeper foundations, heavier steel, and sometimes work to the existing house that wasn’t in anyone’s plan at the start.

Which one suits your house

Mostly the house decides, not you.

A terrace with a side return and a reasonable garden is the classic wraparound candidate. You’ve got the alley to fill, and going up at the back is often restricted by how close the boundary is.

A semi-detached with a deep garden is usually the better double storey candidate. There’s room to extend without swallowing the outside space, and the roof geometry tends to be simpler.

A detached house can generally do either, and has the loosest planning position of the three.

If the extra room you want is a bedroom, going up is the answer. If it’s living space and light, going round is. And if what you actually need is one more bedroom and nothing else, price a loft conversion before you commit to a double storey — on some houses it’s cheaper and it doesn’t touch the garden.

Planning permission, and where the line runs

This is where the two diverge sharply.

Wraparound. Each half might individually fall under permitted development, but combined they usually break one of the conditions — most often the rule about how much of the original garden you can cover. Most wraparounds in London go in as a full planning application. Assume you’ll need one and treat it as a bonus if you don’t.

Double storey. Two-storey rear extensions can be permitted development, but the conditions are much tighter than for single storey. No more than three metres beyond the original rear wall, and at least seven metres between the new rear wall and the boundary opposite. Miss either and it’s a full application. Terraced and semi-detached houses in conservation areas are excluded from the wider rights entirely, and plenty of London is in a conservation area.

There’s also the 45-degree guidance most councils apply to protect neighbours’ light, and the right-to-light question if there’s a window close to your boundary. Neither is a formality when you’re building at first floor level, because that’s the height where you start blocking somebody’s afternoon.

Our planning and design team deals with the applications, and the honest advice is always the same: find out the constraints before the drawings, not after. A design developed around what the council will actually approve costs less than a beautiful one that gets refused.

Building regulations sit separately from all of this, and apply either way.

Structural work you can’t skip

This is the part that separates the two jobs from a straightforward rear extension, and it’s the part that’s invisible once the plaster is on.

Foundations. Existing house foundations are often shallower than modern requirements — on Victorian stock, sometimes dramatically so. Put a second storey on and you may be asking a footing to carry a load it was never designed for. The answer is either new deeper foundations for the extension with a careful junction to the old, or underpinning part of the existing house. Neither is exotic. Both need to be priced honestly at the start. On a rear extension we built in Chiswick, even at single storey the footings had to go deeper than the original before anything could be built on them.

Steel. Longer openings and upper floors mean goalpost frames rather than a single beam — two columns and a beam working together, with padstones sized by the engineer. Steel has a lead time. Order it late and the whole programme waits.

Drainage. On a wraparound, the side alley usually contains the drains. If a public sewer runs under or near your extension you’ll need a build-over agreement from the water company before you start, and that takes weeks. Inspection chambers may need relocating or replacing with an access-friendly design.

The junction with the existing roof. On a double storey this is where water gets in if it’s detailed badly. New roof structure has to tie into old, with flashings and a proper drainage path.

None of this is a reason not to build. It’s a reason to have the survey and the structural design done properly before anyone gives you a fixed price. Our pre-construction stage exists for exactly this.

Costs and build time

Broad London ranges, build cost only, before kitchens, bathrooms and VAT:

Add design and engineering on top, plus planning fees. Add a contingency — on this size of job, 10% is sensible and on an older house it’s closer to essential.

What moves the number is nearly always below ground or behind plaster: foundation depth, underpinning, drainage diversions, steel tonnage. Glazing and finishes move it too, but those you choose. The groundwork you inherit.

Before site there’s design, planning and building regs. Realistically four to six months from first conversation to first dig if planning permission is needed. That waiting period is the single most common reason projects miss the summer everyone was aiming for.

Frequently Asked Questions

How much does a double storey extension cost in London?

Typically £90,000 to £180,000 for the build, depending on floor area, specification and how much structural work the existing house needs. Foundations and underpinning are the items most likely to push a job to the top of that range. Kitchens, bathrooms and VAT sit on top.

Do I need planning permission for a double storey extension?

Often, though not always. Two-storey rear extensions can fall under permitted development if they extend no more than three metres beyond the original rear wall and leave at least seven metres to the rear boundary. Conservation areas and terraced houses in designated areas are usually excluded. Building regulations approval is needed regardless.

What is a wraparound extension?

A side return and a rear extension built as a single project, wrapping around the back and side of the house in an L shape. It’s the largest single-storey addition most London terraces can take, and it usually requires full planning permission rather than permitted development.

How long does a wraparound extension take?

Fourteen to twenty weeks on site for a typical job, with four to six months beforehand for design, planning and approvals. Drainage diversions are the most common cause of delay once work starts.

Is it cheaper to build two storeys at once?

Per square metre, considerably. You pay once for scaffolding, roof works, foundations and the disruption. Coming back to add a first floor later typically costs far more than including it from the start, and means going through planning and building control twice.

Will my neighbours need to agree?

They don’t have a veto, but they do have a say. A planning application is publicly consulted on, and councils weigh objections about light and overlooking, particularly at first-floor level. Separately, the Party Wall Act requires you to serve notice on adjoining owners — two months for most work. Talk to them early. A neighbour who hears about your plans from the council rather than from you is a neighbour with time to object.

Worth a conversation before the drawings

Both of these are jobs where the survey determines the price, and the price determines whether the project happens at all. Finding out that the foundations need underpinning, or that a sewer runs where you wanted your steel, is much cheaper as a conversation than as a variation.

Force Builders Ltd has been building extensions across London for over 25 years. If you’re weighing up going round against going up, call 077 6525 5189 and we’ll tell you what your house will and won’t take. You might also want to read our guide to kitchen extensions in London, since that’s usually what ends up in the new space.