A kitchen extension is the project most British homeowners think about longest before they commit. It's the room they spend the most time in, the one that matters most to how the house feels day to day, and the one that adds the most to resale value when done well. It's also expensive and disruptive. The months of planning are justified: a kitchen extension built on a well-considered brief and a properly developed design is a different outcome from one built on a layout that seemed fine on paper but doesn't work when you're actually in it.
This guide covers the decisions that actually matter: how much space you need and why, the structural implications, design decisions that outlast any kitchen specification, and the sequencing that determines whether you have a working kitchen for most of the build or none at all.
How Much Space Is Actually Needed
The aspiration for kitchen extensions is usually "as large as possible." The reality is that function matters more than size, and the most used kitchens often aren't the largest ones.
The working triangle (the relationship between sink, hob, and fridge) needs to be efficient regardless of how large the room is. A badly planned 40m2 kitchen extension can be less useful than a well-planned 20m2 one. Before thinking about how far to extend, think about what you're trying to achieve: family kitchen with dining space, separate cooking and eating zones, a utility area integrated or separate, connection to garden?
As a rough guide: a kitchen/dining room for a family needs at least 20-25m2 to feel genuinely comfortable with furniture. A kitchen, dining, and informal living space (the "kitchen living" concept) needs 35-45m2 or more. Less than 18m2 and you're cramped unless the layout is very carefully considered.
The proportion matters as much as the area. Long and narrow is harder to furnish and feels confined. A roughly square space, or one where the length is no more than twice the width, usually works better for kitchen living. This influences whether a rear extension, a side return, or a wraparound is the right approach for your house.
Structural Implications
The structural work in a kitchen extension is almost always more than people anticipate. The point of a kitchen extension is usually to create an open connection between the new space and the existing house. That means opening up the existing rear wall. And in most houses, that rear wall is load-bearing.
Removing a load-bearing wall requires a structural steel beam, new padstones in the supporting piers, and temporary propping during the work. The beam sits on the piers either side of the opening and carries the load that the removed wall was previously carrying (the upper floors, roof structure, or both). The size of the beam depends on the span and the load, calculated by a structural engineer.
Getting the structural engineering right is not optional. A beam that's undersized for the load will deflect and crack. A beam that's sat on inadequate padstones will crack the masonry around it. A professional structural engineer's calculations cost £500-£1,500 and prevent problems worth many times that.
The other structural element people underestimate is the new extension foundation. Ground conditions, proximity to trees, drainage runs, and whether the new structure is bearing on made ground all affect what the foundations need to be. If you're extending over or near existing drainage, the pipe positions need to be confirmed before foundations are designed.
Glazing and Light
The most commented-on element of any kitchen extension is usually the glazing. Roof lights, bifold doors, sliding doors, full-width glazed walls: what works best depends on the orientation, the garden relationship, and your priorities.
Roof lights. Adding roof lights to a kitchen extension is one of the most effective ways to introduce daylight into a space that might otherwise be dark toward the middle of the room. In a single storey rear extension, the existing house wall above the extension can shade the new space considerably. A flat roof section or a vaulted ceiling with roof lights overcomes this. Fixed roof lights are cheaper; opening ones provide summer ventilation. For a kitchen, at least one opening roof light is worth having.
Bifold vs sliding doors. The debate between bifold and sliding doors runs constantly in kitchen extension circles. Bifolds fold back to give a wider clear opening (the whole wall can open) but the doors stack in a fixed location when open. Sliders give a cleaner aesthetic in both open and closed positions and don't protrude into the room when open, but don't clear the full opening width. Neither is objectively better: the right choice depends on the specific situation, how you use the garden, and what the design looks like.
South-facing glazing overheating. A south-facing kitchen with lots of glass will get very warm in summer. Solar control glass (with a coating that reduces solar heat gain while still allowing good light transmission) is a worthwhile specification for any significant south-facing glazing, particularly on a roof. The cost premium over standard double glazing is modest relative to the benefit.
Kitchen Specification Within the Build
The kitchen extension build and the kitchen installation are two separate projects that need to be coordinated, not treated as one. The builder builds the shell; the kitchen company installs the kitchen. This is normal but creates interface issues if not managed carefully.
The builder needs to know the kitchen layout before first fix electrical and plumbing. Socket positions, extraction fan routes, gas or electric hob connections, waste pipe positions, and boiler relocation if needed all need to be confirmed before walls are lined. A kitchen that's designed and ordered after the first fix is complete results in compromises and sometimes expensive alterations.
Get the kitchen company involved in the design process early, even if you're not ordering for months. Their input on socket heights, worktop heights, island dimensions, and extraction routes will save problems later.
Plan your temporary kitchen. You'll be without a proper kitchen for 12-16 weeks during the build. A microwave, a kettle, and a plug-in electric hob in a back room is survivable for most families. Booking meals or planning around it is better than having no plan at all. The days when you have no running water for a full day (drainage works, pipe relocations) need to be anticipated.
Costs
Kitchen extension costs have two components: the building work and the kitchen itself. They need to be budgeted separately and together.
| Element | Approximate cost (2025) |
|---|---|
| Extension build (20-25m2, mid-spec) | £55,000 - £85,000 |
| Extension build (30-40m2, mid-spec) | £85,000 - £130,000 |
| Structural steel and opening up (existing wall) | £4,000 - £12,000 |
| Fitted kitchen (mid-range, supply and install) | £15,000 - £35,000 |
| High-end fitted kitchen (bespoke or luxury brand) | £35,000 - £80,000+ |
| Underfloor heating (wet, included in build) | £3,000 - £7,000 |
Professional fees (architect, structural engineer) typically add 12-15% to the build cost. Allow 15% contingency on the build. The kitchen itself is separate and its cost is determined by specification choices, not the building contractor.
Sequencing the Project
The sequence that causes the least disruption is usually: complete the extension shell and all structural work to the existing house first; install services and first fix throughout; apply screed if underfloor heating is included (wait for the screed to dry, minimum 28 days for sand/cement); insulation and boarding; plastering; second fix electrics and plumbing; then bring in the kitchen company for installation.
Kitchens should go in last, after all wet trades are complete and the room has dried out properly. A kitchen installed in a room with residual moisture in the screed or plaster will show movement and gaps in the joinery within months.