Why the kitchen is usually the right room to extend

In most British homes the kitchen is the smallest room doing the hardest work. It is where you cook, where children do homework, where guests end up standing with a glass of wine. A single-storey rear extension solves that in one move, adding roughly 15 to 25 square metres of floor space at the back of the house and connecting the kitchen to the garden.

Going out rather than up also tends to be the better value route. There is no scaffolding, no staircase to rebuild and no need to re-roof the existing house, so the cost per square metre is generally lower than a loft conversion or a two-storey addition. As a rough guide, allow £2,000 to £3,000 per square metre outside London and the South East, more in the capital, and add the kitchen itself on top. What you give up is garden, so think carefully about depth before you commit.

Getting the glazing and light right

Most homeowners are certain about one thing: they want as much glass as possible. It is worth knowing the trade-offs before the design is frozen. A fully glazed rear wall with a roof lantern looks spectacular in July, but an east or west-facing elevation can turn the room into a greenhouse by mid-afternoon. Building Regulations now require overheating to be considered, so plan for a mix of solar-control glass, deeper reveals, external blinds or a brise-soleil, plus openable panels for night purging.

On performance, aim for windows and doors with U-values of 1.4 W/m²K or better and rooflights around 2.2 W/m²K, with the design as a whole meeting the current notional dwelling specification. Sliding or bifold doors need a structural opening wide enough to take the frame and still give you a sensible corner for units. A roof lantern over the centre of the room, rather than tight against the house wall, throws light into the darkest part of the plan.

Drainage and groundworks: the decisions you cannot undo

This is where extensions get expensive and where early surveys pay for themselves. Check the position of every manhole, gully and downpipe, and get the invert levels surveyed. A manhole sitting in the middle of your new footprint has to be either relocated or built over with a sealed, rodable system approved by Building Control, and that can add thousands.

• Foul drainage: new runs fall at roughly 1 in 40 to 1 in 110 for 110mm pipe, so depth and connection points drive the whole floor level.
• Surface water: many areas now expect sustainable drainage, so a soakaway, attenuation crate or water butt may be required rather than a straight connection to the sewer.
• Services: water main, gas, electricity and broadband ducts often cross the extension footprint and must be diverted, protected or re-laid.
• Foundations: near trees, drains or old extensions, expect deeper trench fill or a raft, and allow for a trial hole before the design is finalised.

Structural openings and load paths

Opening up the back of a house means removing a wall that is almost certainly holding something up. A structural engineer will size the steel, and Building Control will want the calculations. Practical points that catch people out include padstones to spread the load into the existing brickwork, a proper load path down to the foundations, and temporary propping during the works.

Where the opening is wide, a goalpost frame of two columns and a beam often works better than one very deep beam, because it keeps head height. A 4m clear opening may need a beam 400mm deep, which eats into your ceiling line and can clash with an extract duct. Decide early whether the kitchen sits under the beam or beyond it.

Planning permission, permitted development and building regulations

Many rear extensions are permitted development, but the rules are specific. For a single-storey rear extension on a terraced or semi-detached house, the limit is usually 3 metres deep and 4 metres high; for a detached house, 4 metres. Under the neighbour consultation scheme you can apply for up to 6 metres on an attached house and 8 metres on a detached one. Eaves within 2 metres of a boundary are capped at 3 metres, and materials must match.

Conservation areas, listed buildings and Article 4 directions remove these rights quickly, so a Lawful Development Certificate is cheap insurance if you want to sell later. Separately, Building Regulations approval is always required, covering structure, thermal performance, electrical work, ventilation and, for kitchens, mechanical extraction.

Budget, programme and living through the build

Allow three to five months for a typical extension, plus lead times of six to ten weeks for glazing and steels. Sequence matters: groundworks, foundations, drainage, slab and insulation, brickwork, roof, then glazing to make the shell watertight before first fix. Order the kitchen only after plastering, and confirm appliance positions with the electrician and plumber beforehand.

Keep walkways at 1.1 metres minimum, leave a straight, short duct route for the extractor, and think about where the boiler and consumer unit will end up. If you are living in the house, set up a temporary kitchen with a microwave, kettle and washing-up bowl in another room — it is grim but survivable. Get the design right at the drawing stage, and the build itself becomes remarkably calm.