The single-storey rear extension to create a large kitchen-dining space is the defining renovation project of London's Victorian terraced stock. Here is what it really involves.
The single-storey rear kitchen extension is the most common renovation project in London. The majority of Victorian and Edwardian terraced houses have an original rear addition — a narrow, dark Victorian scullery — that limits the kitchen to a functionally and spatially inadequate footprint. Replacing or extending beyond this to create a wide, glazed kitchen-dining space transforms how the house works.
It is also one of the most technically complex projects in the domestic renovation sector, requiring coordination across structural engineering, planning, drainage, and multiple specialist trades. Done well, it adds significant value and usability. Done poorly, it creates expensive problems.
What the project involves
A typical rear kitchen extension in a London Victorian terraced house involves:
- —Demolition of the existing rear addition (or part of it)
- —Excavation of foundations for the new structure
- —Construction of a single-storey extension: typically 3–5m rear projection, full width of the property
- —Installation of a structural steel frame or RSJ beam at the house-to-extension junction to open up the rear wall
- —Installation of a rooflight — one or more — in the new flat or pitched roof
- —Installation of glazed bifold or sliding doors across the rear elevation
- —Full internal fit-out: kitchen, flooring, electrical, plumbing, lighting
Planning
A single-storey rear extension falls within permitted development in most cases for non-listed properties outside conservation areas, subject to constraints on:
- —Projection: the extension must not project beyond the rear wall by more than 4m (detached) or 3m (terraced or semi-detached) without prior approval under the Neighbour Consultation Scheme (which allows up to 6m or 8m respectively if no objection is received).
- —Height: must not exceed 4m to the ridge or 3m at the eaves adjacent to a boundary.
- —Materials: in some boroughs, an Article 4 Direction has removed permitted development rights. Check with the local authority.
In a conservation area, permitted development rights for rear extensions are often removed. A full planning application will be required. Most London boroughs will support a well-designed rear extension in a conservation area, but the process takes longer and gives neighbours more opportunity to object.
Structural considerations
Opening the rear wall of a Victorian terrace to the full width typically requires:
- —An RSJ (rolled steel joist) spanning the opening, supported on padstones bearing on the existing party walls
- —Calculation by a structural engineer (required for building control and for the party wall surveyor if applicable)
- —Party wall notices served on both neighbours at least two months before work begins
The structural steel is usually a flitch beam (timber-encased steel) or a plain steel section. Sizing depends on the span and loading above. For a typical three-bedroom terrace, the span is typically 3.5–4.5m, and the beam size is typically 203x203 UC or 254x146 UB — but this must be confirmed by the engineer for each project.
Rooflight design
The most common approach for a flat-roof extension is one or more large rooflights set flush with the roof plane. Pitched glazing (glass spanning from the parapet to the house wall) gives the best natural light but is thermally demanding. Frameless or slim-frame rooflights give the cleanest aesthetic.
Key considerations: - U-value: current Part L compliance requires 1.6 W/m²K or better for rooflight glazing. - Condensation: warm-edge spacer bars and argon fill reduce cold-edge condensation. - Cleaning: rooflights need maintenance access. Self-cleaning glass is worth specifying.
Drainage
The extension sits over part of the original drainage run in most terraced houses. This drainage must be rerouted before the new foundations are poured. Building control requires drainage to be at minimum 150mm below the extension slab, in a duct if within 3m of a load-bearing element. Unexpected drainage complications — cracked or collapsed clay pipes, misconnections, cesspits — are one of the most common sources of cost overrun in London extension projects.
An exploratory drainage survey before the project begins — CCTV the drains — costs around £400–£600 and can identify problems that would otherwise only emerge during excavation.
Realistic costs
| Scope | Approximate cost (exc. VAT) |
|---|---|
| Basic single-storey rear extension, standard spec | £80,000 – £130,000 |
| High-specification extension with large rooflights, bifold doors, quality kitchen | £130,000 – £200,000 |
| Premium extension with bespoke kitchen, marble worktops, underfloor heating | £200,000 – £300,000+ |
These figures include demolition, structure, roofing, glazing, and fit-out including kitchen but excluding landscaping. They assume normal ground conditions.
The fit-out: where the costs accumulate
The shell of the extension — structure, roof, glazing, plastering — is typically 40–50% of the total project cost. The fit-out — kitchen, flooring, lighting, underfloor heating, electrical, plumbing — accounts for the rest. A project managed as two separate appointments (shell then fit-out) rarely goes well; the trades need to be coordinated from the outset so that services are embedded in the slab, electrics are chased before plastering, and the kitchen is designed before the joinery is fixed.
ASAAN manages rear kitchen extensions as whole-project renovations — from planning through to final decoration. If you are considering this project, contact us for a preliminary discussion before committing to a design approach.
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