Open Plan Kitchen-Diner Extension: A London Guide
Title: Open Plan Kitchen-Diner Extension in London: Costs, Design, and Planning Guide Target keyword: kitchen diner extension London
The open plan kitchen-diner extension is one of the most transformative home improvements available to London homeowners. By knocking through the rear of a Victorian or Edwardian terraced house and extending into the garden, you create the kind of light-filled, flexible living space that many buyers are actively looking for.
Here's what you need to know before starting.
What Does the Project Typically Involve?
A kitchen-diner extension is rarely just "building a box at the back." A typical project includes:
- Demolition of part of the existing rear wall to create the opening between old and new
- Structural steelwork (RSJ/universal beam) to carry the structure above the opening
- New extension structure — footings, slab, walls, roof (flat or pitched)
- Connection between old and new — flooring, ceiling, electrics, heating
- Rooflights in the extension roof (very popular — they bring light into the old part of the kitchen when the extension is at the rear)
- Bifold or sliding doors opening onto the garden
- Drainage — especially if relocating the kitchen to a different position than it previously occupied
- Kitchen fit-out — supply and installation of new kitchen (client-supplied units and appliances in our standard model)
Costs
| Extension Size | Rough Build Cost (exc. kitchen) | All-in (inc. design, BCO, contingency) |
|---|---|---|
| Small (12–16m²) | £48,000–£65,000 | £60,000–£82,000 |
| Medium (16–24m²) | £65,000–£90,000 | £82,000–£115,000 |
| Large (24–35m²) | £90,000–£130,000 | £115,000–£165,000 |
Add the kitchen unit cost separately: £8,000–£40,000+ depending on specification, all client-supplied unless otherwise agreed.
Structural Steelwork
Almost every open plan kitchen-diner extension involves at least one RSJ — usually to span the opening in the rear wall. The size of the steel depends on the span and the load above. Your structural engineer specifies it; your contractor supplies and installs it; Building Control inspects and signs off.
Some extensions require a more complex steel package — for example, if a chimney breast is being removed, or if there are loads from an upper floor that need transferring in a new way. This is discovered during the structural engineer's site visit and affects the final cost.
Planning Permission
Many kitchen-diner extensions in London fall under permitted development — meaning no planning application is required. Key limits: - Single-storey rear extension: up to 3m beyond the original rear wall (terraced/semi) or 4m (detached) under standard PD - Up to 6m/8m under the Prior Approval (Neighbour Consultation) scheme - Must not cover more than 50% of the original garden area - Must not exceed the height of the original dwelling
Conservation area caveat: In conservation areas, PD rights for rear extensions may be restricted. Many inner London streets — Clapham, Islington, Hackney, and others — have conservation area designations that affect this. Always check with your LPA.
Even where PD applies, Building Regulations approval is still required — covering structure, drainage, insulation, fire safety (Part B), and ventilation (Part F).
The Build Process
A typical kitchen-diner extension takes 14–22 weeks from start on site to practical completion:
- Weeks 1–4: Groundworks and foundations
- Weeks 5–8: Structure (walls, steelwork, roof)
- Weeks 9–12: First-fix plumbing, electrics, insulation
- Weeks 13–16: Plasterwork, second-fix trades
- Weeks 17–20: Flooring, kitchen installation, decoration
- Weeks 21–22: Snagging and BCO final inspection
The kitchen fit-out is typically toward the end — which is why it's worth having your kitchen supplier and designer confirmed before the build starts, not midway through.
rcbGroup's Approach
At rcbGroup, we manage kitchen-diner extension projects end-to-end — from pre-construction design coordination through to practical completion. We don't outsource the risk. We're your single point of contact.
Call +44 7359 872594 or visit rcbGroup.co.uk to discuss your project.