There is no honest single price for an extension. The cost is set by the size of the floor area, whether it is single or two storey, the ground conditions and foundations, the roof type, how much glazing you want, and what goes inside it, especially a kitchen or bathroom. Professional fees, approvals and the level of finish then add to the build itself. The only reliable figure is a written quote based on your drawings, which is why it pays to understand each cost driver before you ask for prices.
The size and shape of the extension
Floor area is the starting point. A bigger footprint means more foundations, more walls, a larger roof and more floor to finish. But cost does not rise in a neat straight line: a very small extension still needs the same excavation equipment, the same building control visits and the same connection to your existing house, so the fixed costs are spread across fewer square metres.
Shape matters too. A simple rectangle is quicker to set out and build than an L-shape or a wraparound design with extra corners, steels and roof junctions. If you are working to a budget, a clean, simple outline usually gives you more usable space for the money.
Single storey or two storey
A two storey extension adds a lot of space for the extra walls, because the foundations and roof are shared across both floors. However, it usually needs more structural design, scaffolding, a staircase arrangement and, in many cases, a full planning application rather than permitted development. A single storey rear extension is often the simpler route, particularly for kitchen and living space.
Ground conditions and foundations
What is under the ground is one of the biggest unknowns. Clay soil, nearby trees, soft ground, or drains running through the area where the extension will sit can all mean deeper or specially designed foundations. A public sewer close to the build may need approval from the water company before work starts. These things are not always visible until digging begins, so a sensible budget includes a contingency for them.
Roof, glazing and openings
A flat roof is generally simpler than a pitched roof with tiles to match the house. Roof lanterns, skylights and large sliding or bifold doors make a space feel bright and open, but big openings usually need steel beams and the glazing itself is a significant item. Knocking through into the existing house to make an open-plan room also brings structural work, making good and redecoration.
What goes inside
An extension used as a simple living room is very different in cost from one housing a new kitchen, utility room or shower room. Kitchens, plumbing, underfloor heating, electrics, flooring and decoration can add as much as the shell itself. It helps to decide early whether your quote is for a finished room or a weathertight shell that you will fit out separately.
Fees and approvals to budget for
Most extensions need drawings, and many need structural calculations. Building Regulations approval is required for almost every extension, and you may also need planning permission or a lawful development certificate. If you share a wall or are digging close to a neighbour's foundations, the Party Wall etc. Act 1996 may require notices and, in some cases, surveyors. These costs sit outside the building work, so ask whether a quote includes them.
- Architectural drawings and structural calculations
- Planning or lawful development certificate fees
- Building control fees
- Party wall notices or surveyors where needed
- A contingency for unforeseen groundwork
How to get a quote you can rely on
Compare like with like. Give each builder the same drawings and the same list of what you want included, then check what each quote covers: groundwork, drainage, steels, roofing, glazing, electrics, plastering and finishes. A quote that looks cheaper may simply leave items out. Ask how unforeseen ground conditions would be handled and what the payment stages are.
Superior Build & Landscape is run by George Halliwell, with 11 years trading, public liability insurance and a 10-year guarantee. If you are planning an extension anywhere within about 30 miles of our NN6 base, including Northampton, Kettering, Wellingborough and Towcester, George can visit, talk through your plans and put together a quote for your project.
Frequently asked questions
Is a single storey extension cheaper than two storeys?
The total is usually lower for a single storey, but a two storey extension often gives more space for the money because the foundations and roof serve two floors. Planning requirements are also usually simpler for single storey work.
Does a kitchen extension cost more?
Usually, yes. The kitchen units, worktops, appliances, plumbing and extra electrics are on top of the building shell, so a kitchen extension generally costs more than the same size living space.
What hidden costs should I plan for?
Drawings, structural calculations, building control, planning or certificate fees, party wall matters, drainage diversions and unexpected foundation work are the most common items that sit outside the basic build.
How can I keep the cost of an extension down?
Keep the shape simple, fix your design before work starts, avoid changes mid-build, and be clear about whether you want a finished room or a shell you will fit out yourself.