Residential · First Floor Joint Extension

First Floor Joint Extension — Clayhall, London

A rare design brief: two neighbouring homes deciding to extend together. A single coordinated first-floor rear extension, sitting above the existing ground-floor additions on both properties — symmetrical, neighbour-friendly, and far easier to get past planning as a pair than as two separate applications.

Location

Clayhall, London

Properties

Two adjoining terraces

Scope

Joint first-floor rear extension

Status

Planning permission granted

The brief

Two homes, one design conversation.

The owners of two adjoining houses each wanted the same thing: a meaningful upstairs extension above the rear of the home, adding bedroom and bathroom space without losing garden depth.

Rather than design two separate proposals — risking mismatched roofs, party-wall friction and competing planning applications — the neighbours agreed to commission a single, mirrored design.

The result is one cohesive piece of architecture, drawn as a pair, submitted as a pair, and built as a pair: lower cost per household, a stronger planning case, and a back elevation that looks intentional rather than improvised.

Context

A pair within a longer terrace.

· Two adjoining homes within a continuous London terrace.
· Existing single-storey rear extensions of similar depth on both properties — the natural footprint for a first-floor addition.
· Generous rear gardens that absorb the new massing comfortably.

Aerial view showing the two adjoining houses and their existing rear extensions — Aerial context — the existing rear extensions visible across both plots.

What stays

Ground floor and front: untouched.

· No alterations to the existing ground floor plans.
· No alterations to the front elevation — the street keeps its rhythm.
· All new work is contained at first-floor level, at the rear, behind existing structure.

Existing ground floor plans of both houses and front elevation, both retained — Existing ground floor plans & front elevation — both retained as-is.

What changes

A new upstairs, drawn as a pair.

· New first-floor extension across both properties simultaneously.
· Sits squarely above the existing single-storey rear extensions — no new ground footprint.
· Mirrored fenestration and matching roof pitch read as one composition.
· Materials matched to each home's existing render and tile palette.

Existing rear elevation and proposed rear elevation showing new first floor extension — Existing (left) & proposed (right) rear elevations, 1:100 — the new upper level reads as one piece.

Planning analysis

The 45° rule, drawn out properly.

· 45° rule tested from the closest neighbouring habitable windows on both sides — proposal stays clearly below the line.
· No loss of light or outlook to adjoining properties beyond the joint pair.
· Height and depth deliberately moderated to keep the planning case straightforward.
· Result: planning permission granted.

View analysis drawing showing 45 degree rule compliance from neighbouring windows — View & daylight analysis — 45° rule, 1:200.

In summary

"The best neighbour conversation is the one that ends with both of you signing the same set of drawings."

Thinking of extending alongside a neighbour?

A joint planning application can be cheaper, calmer and more likely to succeed. Worth a conversation before either of you starts on your own.

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