A house extension is a permanent addition to an existing home that increases its floor area, built onto or above the current structure rather than as a separate building. It lets a household gain rooms or space without the cost and upheaval of moving, while keeping the same address and neighbourhood.
What counts as a house extension?
An extension enlarges the footprint or volume of a home by adding new built space that connects to the original. This sets it apart from a standalone garden room or a loft conversion, which reworks the roof space you already have.
Most extensions fall into a handful of recognised shapes. The form chosen depends on the plot, the existing layout, and how much extra space is needed.
- Single-storey extension — one floor added to the rear or side, often to create a larger kitchen or living area.
- Double-storey extension — two floors stacked, typically adding a downstairs room plus a bedroom or bathroom above.
- Side-return extension — fills in the narrow alley alongside a terraced or semi-detached house, common in Victorian properties.
- Wraparound extension — combines a rear and a side-return extension into an L-shape, producing a much larger ground floor.
Single-storey, double-storey or wraparound?
A house extension is a permanent addition to an existing home that increases its floor area, built onto or above the current structure rather than as a separate building.
The right type depends on what you want and what the site allows. A single-storey extension is the most common choice. It suits households wanting an open-plan kitchen-diner or a brighter family space at the back of the house.
A double-storey extension delivers far more space for a smaller increase in cost per square metre, because the foundations and roof serve two floors instead of one. It works well where a growing family needs both a downstairs room and an extra bedroom.
A side-return extension makes use of dead space that many older terraces waste — the thin strip of yard beside the kitchen. Filling it in can transform a cramped galley kitchen into a wide, light-filled room.
A wraparound extension is the most ambitious of the four. By extending both backwards and sideways, it can reshape the entire ground floor. It usually costs the most and is more likely to need full planning permission rather than relying on permitted development rights — the rules that allow certain projects without a formal application.
When is one the right choice?
An extension tends to make sense when a household likes where it lives but has outgrown the space. Comparing the cost of building against the cost of moving — including stamp duty, estate agent fees and legal work — often makes staying put the cheaper route to more room.
It is less suitable when the plot is too small to build on without losing all the garden, or when the extra value would push the property well above what similar homes in the street sell for. A surveyor or local estate agent can give a sense of whether the spend is likely to be recovered in resale value.
The decision also depends on disruption tolerance. An extension is a significant building project, and parts of the home may be unusable for weeks. Households who cannot face that may prefer to move instead.
What does the build deliver day to day?
The practical result is more usable space, shaped around how a household actually lives. A rear single-storey extension often becomes the heart of the home — a kitchen open to a dining and seating area, with large doors onto the garden.
A side-return or wraparound brings light into the middle of the house, which older homes often lack. Rooflights and glazed doors can turn a dark interior into a bright one.
A double-storey project changes how the upstairs works too, adding a bedroom, an en-suite or a study without disturbing the existing rooms. Done well, an extension also improves flow, removing the pinch points and awkward corridors that made the original layout feel tight.
What drives extension costs?
Cost varies widely with size, specification and location, so figures from one project rarely transfer to another. Several factors consistently move the total up or down.
- Floor area and number of storeys — more space costs more, though a second storey adds floor area more cheaply per square metre.
- Groundworks — difficult soil, sloping plots, trees or nearby drains can mean deeper or more complex foundations.
- Glazing and finishes — large sliding doors, rooflights and high-end kitchens and bathrooms raise the figure considerably.
- Structural changes — removing load-bearing walls to open up the original house adds steelwork and engineering.
- Professional and statutory fees — architects, structural engineers, planning applications and building control inspections all carry costs.
- Location — labour and material rates differ across the UK, with the South East typically higher.
Most projects also need a contingency fund for unforeseen issues uncovered once work begins, such as outdated wiring or poor existing foundations. Allowing a margin on top of the quoted price helps avoid mid-build stress.
How the work is sequenced
An extension follows a fairly consistent order, though timings shift with size and weather. Knowing the stages helps a homeowner judge progress and plan around the disruption.
- Design and consents — drawings are prepared, and either planning permission or confirmation of permitted development is obtained, alongside building regulations approval.
- Party wall matters — where the work affects a shared wall or sits close to a boundary, neighbours may need formal notice under the Party Wall Act.
- Groundworks and foundations — the site is cleared and foundations dug and poured, with a building control inspector checking key points.
- Structure and weatherproofing — walls go up, steelwork is installed, and the roof and windows make the shell watertight.
- First fix — wiring, plumbing and pipework are run before walls are closed up.
- Plastering and second fix — surfaces are finished, then sockets, radiators, kitchens and bathrooms are fitted.
- Snagging and sign-off — final defects are corrected and building control issues a completion certificate.
From first design to handover, a straightforward single-storey project often runs over several months, while larger wraparound or double-storey builds take longer. A clear contract and written schedule with the builder keep expectations realistic on both sides.
Reviewed: June 2026