2026 kitchen extension costs by size
Kitchen extension pricing in 2026 has settled into fairly predictable ranges across London and the South-East, assuming mid-specification work by an established contractor. The cost per square metre for a single-storey kitchen extension typically falls between GBP 2,800 and GBP 3,800, with the variation driven by structural complexity, ground conditions and site logistics rather than the kitchen itself.
For a small kitchen extension of around 15 m², expect to pay GBP 42,000-GBP 57,000 for the building shell and basic finishes. A medium-sized extension of 20 m² - the most common size we build - sits between GBP 56,000 and GBP 76,000. Large extensions of 25 m² run GBP 70,000-GBP 95,000, while extra-large open-plan schemes of 30 m² or more typically cost GBP 84,000-GBP 114,000. These figures cover the structural shell, weatherproofing, insulation, first and second fix trades, and a standard finish - but they do not include the kitchen fit-out itself, which is a separate and significant budget line.
When comparing quotes, always check whether the kitchen installation is included in the builder's price or treated as a separate package. Some contractors include a basic kitchen allowance; others quote the shell only and leave the kitchen to the client. At RCB Design & Build, we present both elements clearly so you can see exactly where every pound goes.
Kitchen specification levels and what they cost
The kitchen fit-out is where budgets diverge most dramatically. A standard specification - laminate worktops, flat-slab or shaker units from a mainstream range, standard ceramic tiles and integrated mid-range appliances - typically costs GBP 8,000-GBP 15,000 fully installed. This delivers a clean, functional kitchen that will last well and look good, but it won't win any design awards.
A mid-range specification steps up to quartz or solid-surface worktops, soft-close drawers and doors, quality wall and floor tiling, and better appliances from brands like Bosch, Neff or AEG. Budget GBP 15,000-GBP 30,000 for the kitchen fit-out at this level. This is where most of our clients land - the quality difference over standard is substantial and the cost increase is proportionate.
Premium kitchens move into natural stone worktops (granite, marble or dekton), bespoke joinery, designer appliances from brands like Miele, Gaggenau or Sub-Zero, and statement lighting and hardware. At this level, the kitchen fit-out alone runs GBP 30,000-GBP 60,000 or more. The craftsmanship and materials are genuinely superior, but it is worth being honest about whether your property and lifestyle justify this level of investment. We help clients find the right specification for their situation rather than defaulting to the most expensive option.
Structural costs that catch people out
Every kitchen extension that opens into the existing house requires structural steelwork to support the load above the removed wall. Steel beams (RSJs) including design, supply, installation and Building Control sign-off typically cost GBP 3,000-GBP 8,000 depending on span length and load requirements. A single beam over a standard opening is at the lower end; twin beams supporting a first-floor party wall or chimney breast removal push towards the top.
Foundation costs are another area where reality diverges from expectation. On London clay - which covers most of Greater London and large parts of Kent and Essex - foundations need to be deeper than standard to account for seasonal clay shrinkage, particularly near trees. Where mature trees are within influencing distance, foundation depths of 2-2.5 metres are common, adding GBP 3,000-GBP 8,000 over standard strip foundations. If the extension route crosses existing drainage runs, diversions can add GBP 2,000-GBP 5,000 depending on complexity and whether the drain serves multiple properties.
Underpinning existing foundations is the cost that nobody wants to hear about. If the new extension loads affect existing shallow foundations - particularly on Victorian properties with minimal footings - underpinning can add GBP 5,000-GBP 15,000. We commission a full structural survey before quoting so these costs are identified upfront rather than discovered mid-build. Honest pricing is a core part of how RCB operates as FMB and TrustMark accredited contractors.
Glazing, doors and bringing in light
The glazing package is often the single biggest specification decision on a kitchen extension, and it has a dramatic impact on both the feel of the space and the project cost. Bi-fold doors - still the most popular choice for rear kitchen extensions - typically cost GBP 3,000-GBP 8,000 for a 3-4 panel aluminium set, depending on size, threshold detail and manufacturer. Budget aluminium bi-folds from mainstream suppliers sit at the lower end; slimline premium systems from brands like Schuco or Origin push towards the top.
Sliding doors offer a cleaner aesthetic and better thermal performance than bi-folds, with no panels stacking into the room. A quality aluminium sliding door set typically costs GBP 4,000-GBP 10,000. The premium over bi-folds is partly driven by the larger glass panels and heavier-duty running gear. For wide openings, a lift-and-slide system provides the smoothest operation but commands the highest price.
Roof lanterns and roof windows transform the light quality inside a kitchen extension, particularly on north-facing or narrow properties. A standard flat-rooflight (fixed or opening) costs GBP 1,500-GBP 3,500 installed. A framed roof lantern - the pitched-glass structures that sit proud of the flat roof - runs GBP 2,000-GBP 6,000 depending on size and specification. Velux-style roof windows set into the flat roof membrane offer a more affordable alternative at GBP 800-GBP 1,500 each installed. We recommend getting the glazing specification right early in the design process because it affects structural opening sizes, steel beam specifications and the overall project aesthetic.
Professional fees and statutory costs
Beyond the build cost, a kitchen extension project carries a stack of professional and statutory fees that need to be in your budget from day one. Architectural drawings - including existing surveys, proposed plans, elevations, sections and construction details - typically cost GBP 2,500-GBP 5,000 for a kitchen extension. The complexity of the design, the number of revisions needed and whether full planning permission is required all affect this figure.
Structural engineering is essential for any project involving steelwork, foundation design or load-bearing wall removal. Structural engineer fees for a standard kitchen extension run GBP 900-GBP 1,800 and cover calculations, beam specifications, foundation design and connection details. This is not an area to economise - under-specified steels mean Building Control rejection and costly redesign on site.
Statutory costs include the planning application fee of GBP 258 (if full planning permission is required - many rear kitchen extensions fall within permitted development), Building Regulations charges of GBP 600-GBP 1,200 for the inspection regime from foundations to completion, and party wall surveyor fees of GBP 900-GBP 1,500 per affected neighbour where the Party Wall Act applies. For a typical kitchen extension project, professional and statutory fees combined add GBP 8,000-GBP 16,000 on top of the build cost. At RCB Design & Build, we present a full all-in project budget at the review stage so there are no hidden costs waiting downstream.
How to budget honestly for a kitchen extension
The most common budgeting mistake on kitchen extensions is treating the builder's quote as the total project cost. It is not. A realistic total budget needs to account for the build shell, the kitchen fit-out, all glazing and door upgrades, professional fees, statutory charges, and a contingency for the unknowns that every construction project carries.
For a medium-sized 20 m² kitchen extension at mid-range specification, a realistic all-in budget in 2026 looks like this: building shell GBP 56,000-GBP 76,000, kitchen fit-out GBP 15,000-GBP 30,000, glazing upgrades GBP 5,000-GBP 12,000, professional fees GBP 5,000-GBP 8,000, statutory costs GBP 2,000-GBP 4,000, and a 10% contingency on the build cost. That puts the total project budget at roughly GBP 90,000-GBP 140,000 all in. It is a significant investment, and it deserves a realistic budget from the outset.
At RCB Design & Build, our approach is to walk clients through every cost line at the project review stage - before any commitment is made - so the total number is understood and agreed. No one should be surprised by the final invoice on a properly managed project. As FMB members and TrustMark registered contractors, transparent pricing is fundamental to how we operate. If you are considering a kitchen extension and want an honest assessment of what it will cost for your specific property, our initial project review is free and without obligation.
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