House Extension Architects in London

London extensions cover a wide range of work — single-storey rear extensions, side-return infills, wraparound additions, two-storey extensions, roof extensions, loft conversions and basements — and the best option is rarely obvious at the outset. A side return often unlocks more usable kitchen-dining space than a deep rear extension; a wider but shallower rear extension may protect more garden and daylight; a two-storey addition gains a bedroom and bathroom but engages neighbours and party walls more heavily. We test the realistic options against your house, your budget and how you actually want to live before committing to a direction.

Many single-storey rear extensions to a house can, in principle, proceed under permitted development — broadly up to 3m beyond the original rear wall for a terraced or semi-detached house and 4m for a detached house, or 6m and 8m respectively under the larger-home prior-approval route, subject to height limits, materials and a neighbour-consultation step. But permitted development cannot be assumed: it does not apply to flats and maisonettes, is removed on listed buildings, is frequently curtailed by Article 4 directions in conservation areas, and is lost where earlier extensions have already used up the allowance. We confirm the position for your specific property, and where works are genuinely permitted we recommend a Lawful Development Certificate so the lawfulness is documented for a future sale.

Whether or not an extension needs permission, the most common reason householder schemes are refused or contested is impact on a neighbour. Councils assess this using the BRE daylight and sunlight guidance and rules of thumb such as the 45- and 25-degree lines taken from neighbouring windows, alongside concerns about overlooking, loss of outlook and a sense of enclosure. We design the depth, height and roof form of an extension to stay within these tests where we can, and where a scheme pushes them we say so honestly and shape the planning strategy around it, rather than discovering the problem at decision stage.

Almost every extension involves new structure — steel beams over wide openings, new foundations, and sometimes underpinning — which a structural engineer must design and calculate. Where that work is on a shared wall, on the boundary line, or involves excavation within three (and in some cases six) metres of a neighbouring building, the Party Wall etc. Act 1996 applies and notices must be served, with statutory notice periods of one or two months. Left to the last minute, party wall procedures routinely delay a start on site. We coordinate the engineer alongside our drawings and prepare the information your party wall surveyor needs, in good time.

Planning drawings show what a building will look like; they are not enough to build from or to price reliably. After permission we prepare the building regulations drawings and the technical and tender information a contractor needs — construction details, specifications and coordinated structural and services information — so that builders price the same defined scope rather than guessing, and the build proceeds from clear instructions. This is where a well-run project controls cost and avoids the disputes that come from pricing a build off planning drawings alone. We do not promise permitted development or guarantee planning approval; we give a proposal the best-prepared case and build it on honest advice.

Extension quotes are notoriously hard to compare, because two fee proposals that look similar can cover very different amounts of work. The first thing to check is scope: does the fee run only to planning drawings, or all the way through building regulations and a tender package a contractor can actually price from? A low figure that stops at planning leaves the expensive technical stage still to come. Next, look at what is excluded — most design fees sensibly exclude council application charges, the structural engineer, party wall surveying and any specialist reports, but you want those exclusions stated, not buried. Ask whether the fee is fixed or a percentage of build cost, how many design revisions are included, and who coordinates the other consultants. Finally, weigh the drawings themselves: a cheap planning-only set that a builder cannot price, or that invites a refusal, is rarely the economy it appears. We set our scope out in fixed stages so you can see exactly what each fee buys.

An extension is rarely the work of the architect alone, and knowing who else is involved helps you budget and avoid gaps. Almost every scheme needs a structural engineer to design the beams, columns and foundations, and a measured survey of the existing house to draw from accurately. Where work sits on or near a boundary, a party wall surveyor serves notices and, if needed, agrees an award — a role separate from the design team. Building regulations bring further input: an energy assessment for the new fabric, and on some projects a build-over agreement with the water authority where you build near a public sewer. More sensitive schemes may add a planning consultant, an arboriculturist where protected trees are close, or a heritage statement on a listed or conservation-area property. The value of a single point of coordination is that these strands arrive in the right order and talk to each other. We appoint and coordinate the consultants a scheme genuinely needs, and tell you which costs sit outside our fee.

A London house extension is a sequence, not a single drawing. First we confirm designations for the address — conservation area, Article 4, listing, and which borough owns the pin. Then we choose the route: householder application, lawful development certificate, prior approval, or listed building consent where relevant. Design freezes depth, height and roof form against neighbour amenity and host character before kitchens are detailed. Planning drawings make the public-law case; after permission, building regulations drawings, engineer calculations, party wall notices and any freeholder consent make the scheme buildable.

What officers assess is documented in our guide to planning officer tests on house extensions: validation completeness, development-plan policy, conservation character, daylight/outlook/privacy, and technical constraints that change the external form. Borough process packs on our Camden, RBKC and Westminster planning hubs spell out how those tests land differently by authority — including Camden’s basement BIA audit path and RBKC’s garden-extent basement limits — so you do not import a W8 assumption onto an NW3 plot.

Without promising outcomes, schemes that read as subordinate to the host, respect rear building lines and roof grain, use honest materials, and prove neighbour relationships in section are more defensible than maximum-volume boxes that ignore terrace pattern. Front-elevation change on Article 4 streets, overbearing flank walls on short gardens, and roof forms that read as an extra storey are common reasons schemes need redesign.

Basements are a separate machine: both Camden and RBKC require planning via Article 4, and both demand technical evidence. Design the policy-compliant envelope first; interiors second. If a proposal needs pre-application advice, we say so before you pay for a full application that is still testing principle.