What a House Extension in London Involves
When people say "house extension" in London they usually mean one of four distinct jobs, and the distinction matters because it changes the planning route, the structural work, and the price per square metre. A single-storey rear extension is the most common: extending the back of the house into the garden, typically for a bigger kitchen-diner, up to 3m for a terrace or semi under permitted development or up to 6m via the Larger Home Extension Scheme prior approval route. A side-return extension fills in the narrow alley down the side of a Victorian or Edwardian terrace, usually combined with the rear extension to create a full-width wraparound, which is where most of the light and space gets added on these houses. A two-storey side or rear extension adds a bedroom or bathroom above the ground-floor addition, and is often better value per square metre than a single-storey box because the same foundation and roof cost is spread across two floors of new space, though it also adds a second structural opening and a roof tie-in at first-floor level instead of ground level. The common thread across all four is that you're not just adding floor area - you're forming a new structural opening into a house that's often 100+ years old and wasn't designed with that opening in mind, which is why the structural engineering and Building Control elements matter as much as the brickwork and finishes. Loft conversions are a related but separate job with different Building Regulations triggers around means of escape and headroom, and we scope that separately rather than folding it into an extension quote. VAT applies to labour and materials on all of this, and it should appear as a clear line item, not buried in a lump sum.
Why Victorian, Edwardian and Post-War Terraces Change the Job
A large share of London's housing stock is Victorian or Edwardian solid-wall terrace, built without a cavity and often without a reliable damp-proof course by modern standards, and that changes how an extension has to be built onto it, not just what it looks like. The party wall you're building against is typically a shared, load-bearing brick wall on a shallow strip foundation, often considerably shallower than a modern extension's foundation would be designed to, and one that was never engineered for a modern extension's loads or for excavation running alongside it - which is exactly why the Party Wall etc. Act 1996 exists for adjacent excavation within 3m or 6m of a neighbour's foundation. Rear additions on these houses also routinely reveal problems only once you open the wall up: loose or corroded wall ties, historic cracking from long-settled movement, or a foundation shallower than drawings assumed, any of which can force a design change mid-job. Side-return extensions in particular usually need some degree of underpinning to the existing party wall foundation because the new extension's floor level and foundation depth don't match what's already there. Ex-council maisonettes and 1930s semis behave differently again - often with cavity walls and more standardised foundations, but bringing their own issues around shared freehold consent, leasehold consent requirements, and sometimes concrete-frame or non-standard post-war construction that behaves quite differently to solid brick when you cut a new opening. Conservation area status, which covers large parts of inner and outer London, can also remove permitted development rights entirely, meaning what looks like a straightforward rear extension needs full planning permission because of the borough's Article 4 Direction.
What Drives the Cost of a London Extension
Cost per square metre on a London extension isn't driven by finish quality as much as people assume - it's driven by structural complexity, access, and ground conditions. Kitchen extension cost in London runs roughly £3,000-£5,000/m² for a single-storey rear build with straightforward garden access; an 18-20m² job commonly totals £55,000-£95,000 including VAT, the structural engineer's fee, and Building Control charges. Side return extension cost in London sits higher, roughly £4,500-£5,500/m², because you're often underpinning the existing party wall foundation, materials have to come through a narrow side passage rather than a wide rear garden, and the steel spanning the full width of the house is longer and more expensive than a simple rear box needs. A two-storey extension is often cheaper per square metre than a single-storey one, roughly £2,800-£4,200/m², because the foundation and roof - the two most expensive elements - are shared across double the floor area; total project costs for a two-storey job commonly run £120,000-£200,000+ depending on size and specification. On top of the extension shell, forming the structural opening between the old house and the new space with an RSJ steel beam typically adds £1,800-£4,500+ depending on span and whether padstones or additional support are needed, consistent with what we quote for standalone knock-through work. Party wall process, where required, adds a further £900-£3,000 depending on whether a single surveyor is agreed or each side appoints their own. Fit-out level on top of all this - kitchen units, flooring, glazing specification - can move the final number substantially, which is why two extensions of identical footprint can land at very different totals.
Permitted Development or Full Planning Permission
Most single-storey rear extensions in London fall under the Town and Country Planning (General Permitted Development) (England) Order 2015, which allows a rear extension up to 3m deep for a terraced or semi-detached house, or 4m for a detached house, without a full planning application, subject to height and eaves limits. Go beyond that, up to 6m for a terrace/semi or 8m for a detached house, and you need prior approval under the Larger Home Extension Scheme, which runs as a lighter-touch neighbour consultation process through the council rather than a full application, with a statutory determination period of around six weeks. Side extensions have tighter limits under permitted development, generally single-storey and no wider than half the original house width, which is why most side-return jobs on terraces get combined into a full planning application alongside the rear element instead of trying to split the two. Conservation areas, and Article 4 Directions that specific boroughs apply on top of them, can remove permitted development rights for extensions entirely, meaning even a modest single-storey rear addition needs a full application with an 8-week (or longer, for larger schemes) decision timeline. We check this at the first site visit against your specific borough's local plan and any conservation area or Article 4 status before pricing, because the planning route affects the programme more than the build itself does.
Building Control: Full Plans or Building Notice
Separately from planning permission, every extension needs Building Control sign-off under the Building Regulations 2010, and there are two routes to get it: a Full Plans application, where detailed drawings and structural calculations are submitted and formally approved before work starts, typically taking 5-8 weeks; or a Building Notice, where you can start on site as little as 2 days after submission but without prior approval of the design, meaning any problems get picked up on site inspections instead of on paper beforehand. For a straightforward single-storey rear extension with a well-understood structural opening, a Building Notice can be the faster, lower-friction route. For anything with more structural or ground-condition risk - a side-return with underpinning, a two-storey addition, unusual soil conditions, or a design close to the boundary - we generally recommend Full Plans, because getting the steel sizing and foundation design checked and approved before you dig avoids a stop-work order mid-excavation if Building Control isn't satisfied. Part A covers structural stability, Part L sets the thermal performance targets for the new walls, roof, windows and doors and caps how much of the extension's floor area can be glazed, and Part H4 requires water company approval for any foundation within 3m of a public sewer. We decide the route with you at the design stage based on the specific job, not as a default answer.
Party Wall etc. Act 1996 on a Shared Terrace or Semi
If your extension involves a shared party wall - which almost every Victorian or Edwardian terrace or semi-detached extension does - or excavation within 3m or 6m of a neighbour's foundation depending on how deep your new foundations go, the Party Wall etc. Act 1996 requires you to serve formal notice before work starts. For work directly on the party wall itself you need two months' notice; for adjacent excavation it's one month. If your neighbour doesn't respond or dissents, you end up in a schedule of condition survey and a formal Award process with a party wall surveyor. Party wall surveyor cost in London typically runs £900-£2,000 where both sides agree a single surveyor, rising to £2,000-£3,000+ if each side appoints their own. This cost is borne by you as the building owner carrying out the works, not split with the neighbour. Starting groundworks before the notice period has run, or without an Award in place where one's needed, leaves you exposed to an injunction that can stop the job entirely and is one of the most common causes of a stalled extension in London. On extension jobs we serve the notices and coordinate the surveyor as part of the build programme, rather than as an afterthought once excavation is already close to the boundary.
How Long a London Extension Takes, Start to Finish
From first site visit to final sign-off, the pre-construction phase - the measured survey, design, structural engineering, deciding the planning route, the Building Control submission and any Party Wall Act notice period - commonly takes in the region of 2-4 months before groundworks even start, and this is where projects most often get compressed unrealistically in initial planning. Once on site, a straightforward single-storey rear extension under permitted development with a Building Notice typically runs somewhere around 12-16 weeks as a planning guide, once the design is settled and the structural engineer's calculations are in hand, though exact timing depends on your specific design and site conditions. A side-return or wraparound extension on a terrace usually adds a further 2-4 weeks to that build programme because of the underpinning work to the party wall foundation and the narrower access for materials and skips. A two-storey extension typically runs longer again, in the region of 16-22 weeks on site, because of the additional floor structure, the roof tie-in at first-floor level, and doubled first and second fix. None of these figures include a full planning application if one's needed, which can add roughly 8 weeks or more before groundworks start - and the Party Wall Act's two-month notice period (one month for adjacent excavation) runs in parallel with design and Building Control preparation where it's handled properly, instead of being tacked on afterwards. Served at the same time as your Building Control application it costs you no extra time overall, but served late, or if a neighbour appoints their own surveyor and negotiations drag, it can add real weeks to a job that's otherwise ready to start. Weather affects roofing and groundworks stages more than any other phase, which is why we sequence those for drier months where the programme allows it.
Where Extensions Go Wrong After Handover
The two most common defects we see on extensions built by other firms both show up well after handover, which is what makes them expensive to fix. The first is cold bridging at the junction where the new extension's roof or wall meets the original house - most often where a steel lintel or beam crosses a masonry wall, or at the wall-to-roof and wall-to-floor abutments - which doesn't show up until the first or second winter, when it appears as a stripe of mould or condensation tracking along the junction because insulation continuity wasn't maintained through the detail. We check and photograph this junction detailing at framing stage, before it's boarded over and impossible to verify. The second is a bridged damp-proof course, where the new extension's floor slab, external render, or a raised patio built up against the original wall isn't lapped correctly with the existing DPC, or ends up above the 150mm minimum clearance below it, so damp tracks up the original wall internally regardless of how well the rest of the extension is built. Other recurring issues include undersized foundations on London clay where a trial pit wasn't dug before pricing, steel beams sized by guesswork instead of calculation - one of the most common Building Control rejection points on knock-through openings - drainage clashes where an existing combined or shared drain runs under the proposed footprint and wasn't surveyed first, and poor flashing at the roof tie-in on side-return and wraparound jobs, which shows up as a leak the first time it rains hard.
Drainage Surveys and Build-Over Agreements with Thames Water
A detail that catches a lot of homeowners out mid-project: if your proposed extension footprint sits within 3m of a public sewer, or within 1m of a public lateral drain, Building Regulations Part H4 requires formal approval from the water company - a build-over agreement, usually with Thames Water in London - before foundations can be dug, on top of and separate from your Building Control approval. Many Victorian and Edwardian terraces have combined drains running under the rear garden roughly where a rear extension footprint naturally falls, sometimes shared with the neighbouring property, and the exact route often isn't obvious from the surface. We commission a CCTV drain survey at the design stage specifically to catch this before foundations are priced, because discovering a drain run under your footprint once excavation has started means redesigning the foundation, diverting the drain, or repositioning the extension - all of which cost far more mid-dig than they would have on paper. A build-over agreement application typically adds a few weeks to the pre-construction programme, which is manageable if it's planned in from the start alongside the Party Wall notice period and Building Control application, but becomes a genuine hold-up if it's only discovered once the trial pit is already open.