Kitchen Diner Extension Cost
A kitchen diner extension is two jobs sold as one. There is the new room on the back, priced by the square metre, and there is the work inside the old house: taking out the wall between kitchen and dining room so that cooking, eating and sitting share one space. Budget for both.
£45,000to£65,000
For a 25 m² extension, before the knock-through inside. Building work at a standard to good finish, with VAT. A guide, not a quote.
Kitchen diner extension prices by size
Building work for the new floor area only, including VAT, at 2026 national rates. The work inside the existing house is a separate line.
| Size | Floor area | Standard to good finish | High finish |
|---|---|---|---|
| 4 m by 5 m | 20 m²Table for six beside the units | £36,000 to £52,000 | £52,000 to £60,000 |
| 4 m by 6.25 m | 25 m²Island and table for six | £45,000 to £65,000 | £65,000 to £75,000 |
| 5 m by 6 m | 30 m²Island, table and a reading corner | £54,000 to £78,000 | £78,000 to £90,000 |
| 5 m by 8 m | 40 m²Kitchen, dining and a sofa end | £72,000 to £104,000 | £104,000 to £120,000 |
Where these figures come from
- Planning Portal: planning permission for extensions
- GOV.UK: Approved Document B, fire safety
- GOV.UK: Approved Document F, ventilation
- HMRC: Buildings and construction (VAT Notice 708), for the 20% rate on extensions
- GOV.UK: Fees for planning applications in England
- GOV.UK: Permitted development rights for householders, technical guidance
- GOV.UK: Building regulations approval
- GOV.UK: The Party Wall etc. Act 1996, explanatory booklet
The knock-through is priced separately
The per m² rate buys new floor. It does not cover what happens to the rooms you already have, and in a kitchen diner that is a real share of the work. The wall between the old kitchen and dining room is often load-bearing, so it needs an engineer's design, a steel beam, temporary propping and a building regulations application of its own. Then the two floors have to be brought to one level, radiators and sockets moved, and the whole combined room replastered. Taking out a load-bearing wall and fitting a steel beam typically costs £1,750 to £4,000 on its own, including VAT.
Ask builders to show that work as its own line. It lets you compare the extension rate between quotes fairly, and it shows what you would save by keeping part of the wall as a wide opening or a pair of piers.
How big does a kitchen diner need to be?
Work from the furniture. An island needs about a metre of clear floor on every side to be usable, and a table for six with chairs pulled out takes roughly 3 m by 2.5 m. Put those two in a row with a run of units behind and you need a room close to 4 m wide and 7 m long, counting the old kitchen. That is why 20 to 25 m² of new floor is the size most families end up at.
Past 30 m² the extra buys a third zone: a sofa, a desk or a play corner. It also buys a deeper plan, which needs rooflights and usually underfloor heating, because there is no longer enough wall for radiators.
Open plan and the fire rules
Opening up the ground floor can change how people upstairs would get out in a fire. If the stairs come down into the new open room, or a bedroom can only be left through it, building control will ask how the layout meets Approved Document B. The usual answers are keeping a door between the hall and the kitchen diner, fitting escape windows upstairs, or adding linked smoke and heat alarms. Settle this at drawing stage. It is cheap on paper and expensive once the walls are down.
Extraction, heating and light
A kitchen that shares air with the sofa needs proper extraction. Approved Document F asks for 30 litres a second from a hood next to the hob, or 60 elsewhere in the room, ducted to outside. A recirculating hood does not count. On an island, that duct has to run through the ceiling to an outside wall, so decide where the hob goes before the roof joists are fixed.
Heating one large room evenly is easier from the floor. Underfloor heating in the new screed is part of our good finish level. Lighting wants at least three circuits (worktops, table, general) so the room can change from cooking to eating at a switch.
Planning and permissions
The extension follows the usual rules: in England, permitted development to 3 m deep on a terraced or semi-detached house and 4 m on a detached one, or 6 m and 8 m with prior approval (£249). A kitchen diner of 25 m² or more often goes beyond the first limit, so plan for the prior approval period or a householder application (£548). The internal knock-through never needs planning permission unless the house is listed, but it always needs building regulations approval. In Scotland both parts need a building warrant.
Kitchen diner costs, answered
How much does a kitchen diner extension cost?
For 25 m² of new floor, allow about £45,000 to £65,000 for the building work with VAT at a standard to good finish. Add the knock-through inside the house, the kitchen and the fees. The calculator on the home page totals the last two for you.
Can I knock through without extending?
Yes, and it is far cheaper. Joining an existing kitchen and dining room needs a beam, building regulations approval and redecoration, but no foundations or roof. It is worth pricing first; some households find the combined room is big enough.
Where should the dining table go?
Nearest the garden doors in most plans, with the kitchen in the middle of the house where it needs less daylight and the services already are. That keeps the sink, the boiler and the soil stack close to where they started, which saves money.
Other cost guides
- Kitchen extension costThe main guide, the price board and the calculator.
- Rear extension costThe straight extension across the back of the house, by depth and width.
- Side return extension costFilling the alley beside a Victorian or Edwardian kitchen.
- Small kitchen extension costJobs under 15 m², and why they cost more per square metre.
Get a quote for your kitchen diner extension
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