How much does an extension cost in Kent (2026)?

There is no single £/m² number for an extension in Kent. Anyone who gives you one is either selling a product (kitchen company, modular extension provider) or hasn't built much. The actual cost of your extension depends on four big variables and a dozen small ones — and the spread between the cheapest and most expensive specification for the same nominal floor area is comfortably 3x.

What this guide does is set out the cost drivers honestly, give current (2026) industry-typical bands for the High Weald and surrounding Kent villages, and flag the costs that homeowners routinely forget when they're budgeting.

Size matters less than people think. Below about 40 sqm, fixed costs (foundations, M&E connection, roof structure, scaffolding, kitchen fit-out, building-control inspections) dominate the budget — meaning a 15 sqm extension and a 25 sqm extension often cost surprisingly similar amounts. Above 40 sqm, you're paying per square metre on the marginal area.

Specification is the biggest single lever. The same 25 sqm extension can be built to a £55k spec (block-and-render, standard double-glazed casements, B&Q kitchen) or a £180k spec (oak frame, structural glass, Plain English bespoke kitchen, polished concrete floor with underfloor heating). The shell is similar; the finishes triple the budget.

Ground conditions in the Weald are the third driver, and the one homeowners least expect. Wealden clay is heavy and shrinks/swells with seasonal moisture. Foundations typically need to go down 2.0-2.5m, sometimes deeper near mature oaks or established hedgerows. Strip footings rarely work; trench-fill or pile-and-beam is the norm. Allow £8,000-£25,000 extra over what an equivalent extension might cost on chalk or gravel.

Planning and conservation constraints are the fourth. A like-for-like rear extension on a 1970s house outside any designation is the cheapest end. A listed Wealden cottage inside the conservation area inside the High Weald National Landscape is the expensive end — three layers of consent, conservation officer involvement, like-for-like material sourcing, slower programme, higher professional fees.

These ranges represent what we and other established builders in TN17/TN18/TN27/TN30 are quoting through 2026. They exclude VAT, professional fees (architect, structural engineer, planning consultant), and kitchen costs beyond carcasses.

• Small single-storey rear extension (12-20 sqm), mid-spec finish: £55,000-£95,000
• Larger rear or wraparound (25-40 sqm) with structural glazing + integrated kitchen: £95,000-£180,000
• Side-return infill on a Victorian semi: £45,000-£85,000
• Two-storey rear extension on a period property: £160,000-£300,000
• Listed-building rear extension (any size, full consents): typically 25-40% more than the equivalent unlisted spec

VAT (20%) applies to almost all extension work on existing residential property. New-build dwellings and listed-building approved alterations can sometimes recover VAT — talk to a VAT-savvy accountant before assuming.

Professional fees typically add 8-15% on top of the build cost: architect (4-8%), structural engineer (£800-£2,500), party-wall surveyor (£400-£1,500 if applicable), planning consultant (£2,000-£8,000 if needed), building-control application (£500-£1,500).

Kitchen fit-out is almost always a separate cost. A builder's quote will usually include the kitchen carcasses but not appliances, worktops, splashbacks, or design fees. Allow £15,000-£60,000 on top of the build budget for a kitchen replacement done at the same time as an extension.

Furnishing the new space: blinds, lighting fittings, furniture, garden landscaping after the builders leave — easily another £5,000-£25,000.

Temporary accommodation if you can't live in the house during the works: budget £1,500-£4,000/month if you're renting a local cottage for 3-6 months.

Conservation area: planning officers require materials that read as part of the village character — peg-tile or plain-tile roofs (no concrete tiles), painted timber casements (no UPVC), brick or weatherboarding (no render unless original to the building). This typically adds 8-15% to the materials budget.

Listed building: every alteration to fabric needs Listed Building Consent. The application process adds 8-13 weeks. Materials sourcing (reclaimed peg tiles, hand-made bricks to match, lime mortar instead of cement) adds 15-25%. Programme is longer because work proceeds in dialogue with the conservation officer.

High Weald National Landscape: design and materials guidance tighter than standard. Approval more likely when an architect with HWNL experience leads the design.

Tree preservation orders (TPOs): common in Cranbrook and Tenterden conservation areas. If your extension footprint is within the root protection area of a TPO'd tree, you'll need an arboricultural assessment and potentially pile-and-beam foundations to avoid root damage — adds £8,000-£20,000.

Distance from mains services: outlying properties south of Rolvenden, around Smarden or out toward Bethersden may need significant runs of new water/electric/gas mains. £150-£300 per metre is a reasonable allowance.

The most authoritative source for UK build costs is the RICS Building Cost Information Service (BCIS), which publishes quarterly cost data based on actual tendered projects. BCIS is subscription-only but RICS members can access it, and architects routinely cite BCIS rates in feasibility studies. Federation of Master Builders publishes an annual Cost of Building Survey free to read. Build It magazine and Homebuilding & Renovating publish indicative ranges aimed at homeowners.

When budgeting at feasibility stage, our usual advice is: take the BCIS / FMB figure for your build type, add 15% for South-East premium over the national average, add another 10-25% if your project sits in any combination of conservation area / listed / AONB, and treat the result as a working budget. Then commission one or two written quotes from established local builders against a typed scope and use those for the real number.