Worked example: 20m² kitchen extension · model kitchen fit-out selected · UK average
Quick budgeting answer: Our UK-average model places this example at £91,500, with lower and higher construction-rate scenarios of £70,000–£114,500. These are planning allowances, not a narrow predicted contractor quote.
Illustrative model results for the precise example shown, not current quotes from builders. Budgets include an assumed 20% VAT on construction, professional/approval allowances and 10% contingency. A survey or different specifications could change them.
The kitchen fit-out is a separate decision
In the scenario above, the calculator includes its middle-case £12,500 supply-and-fitting allowance for a new kitchen. This is a model input, not a market survey or a guaranteed kitchen quote. It sits on top of the extension’s basic construction budget and is adjusted across the lower and higher scenarios.
| Cost component | What to ask |
|---|---|
| Building shell | Foundations, walls, insulation, roofing, external glazing and basic finishes |
| Open-plan structure | Who designs and prices steels and any temporary support to remove the existing rear wall? |
| Kitchen installation | Are cabinets, worktops, appliances, sink, extraction and fitting specified separately? |
| Services | Are electrical circuits, plumbing, waste pipes, heating and ventilation included? |
Three layouts that change the scope
Often combines a wider opening into the old house with space for a dining table and storage.
Needs enough circulation around the island and coordinated floor power or plumbing if required.
May gain width without extending far into the garden, but roof junctions, drains and neighbours can complicate it.
Adds partitions, drainage, cupboards and services beyond an open room, so itemise the additional work.
Think through the disruption
How will the household cook while the existing kitchen is disconnected? Ask for a realistic sequence covering demolition, temporary weatherproofing, first fix, plastering, flooring and the kitchen installer’s arrival. If an existing boiler, consumer unit or soil pipe needs moving, identify that before contractors price the job.
A shell-only number can look tempting beside an all-in quote. Ask for two totals: one for the building work and one for the room ready to use, with the exact kitchen specification attached. Our rear extension guide explains other exclusions to check.
Calculate your own extension budget
The examples above describe specific assumptions. Change the dimensions, region, finish and extras to explore your project.
Calculate my extension price →Common questions
Does the price include a new kitchen?
In this worked example, yes: the model adds a £12,500 middle-scenario kitchen supply and fitting allowance before subsequent percentage allowances. Actual cabinetry, appliances and layout may cost considerably more or less.
Is the existing kitchen removal included?
It depends on the builder specification. Demolition, temporary cooking arrangements, moving services and making good the original room should be itemised before accepting a quote.
Could I keep my existing kitchen units?
Sometimes, if the units suit the new layout and survive dismantling. Confirm compatibility and labour costs with the installer before making savings assumptions.
Pricing context: Federation of Master Builders construction benchmark; the full-budget numbers are generated by our published illustrative model, which makes additional assumptions. For permissions in England see Planning Portal. Other UK nations have different rules. Always obtain site-specific professional advice and comparable written builder quotes.