What the job involves
A single storey extension adds ground floor space to the back, side or occasionally front of a house. With a shell and finish scope, the builder takes it from bare ground to a finished, usable room. That is more than the structure alone.
The usual order of work is site set-up and protection, clearing the area, setting out, excavating and pouring foundations, laying drains, building the floor, raising the walls, putting on the roof, and fitting windows and doors so the building is weathertight. Next the new opening into the existing house is formed, with a structural beam if a wall is coming out. Inside, the work covers electrics, heating, insulation, plastering, flooring, joinery, decorating and making good. Kitchens, bathrooms and specialist finishes are often separate, so check exactly where the builder's scope stops.
The work normally needs building regulations approval, and some extensions also need planning permission. Many modest extensions can go ahead under permitted development, but that depends on your property and location, so confirm it with your local authority before work begins.
What affects the price
No two extensions cost the same. These are the factors that move the figure most:
- Size and footprint. More floor area means more foundations, brickwork, roof and finishing. Cost does not rise in a perfectly straight line, though, because fixed items such as the structural opening and the services connections are needed whatever the size.
- Ground conditions. Clay, tree roots, made-up ground or a high water table can mean deeper or reinforced foundations. This is often the biggest unknown until the trench is dug.
- Drains and services. If a drain or manhole runs through the footprint, it must be diverted or protected. Moving a gas meter, boiler or consumer unit adds work.
- Roof type. A flat roof is simpler than a pitched roof. Roof lights, lanterns, parapets and tiled finishes all add labour and materials, and a larger or more complex roof means more scaffolding time.
- Glazing. Large sliding or bifold doors and big fixed panes cost far more than standard windows. They also need heavier structural support above them.
- Structural work. Removing a load-bearing wall needs an engineer's calculations, steelwork and temporary support. A wider opening means a heavier beam and more making good.
- Specification. Brick type, matching existing bricks, render, insulation levels, underfloor heating, flooring and joinery all vary widely in price.
- Access. Tight side access, no space for skips, or having to carry materials through the house slows the job and adds labour.
- Electrics and heating. Extra circuits, a new radiator run or a consumer unit upgrade add cost. Electrical work must meet Part P requirements where it applies, and gas work must be done by a Gas Safe registered engineer.
- Professional fees. Architect or designer drawings, structural engineer, building control fees and any party wall surveyor fees are separate from the builder's price.
- Location and demand. Labour rates and builder availability differ across the UK, and a busy builder with a full diary may price higher.
The prices on this page show what homeowners actually paid for this kind of work, which gives you a more useful reference than a national average.
Ways to keep the cost down
- Keep the design simple. A plain rectangular footprint with a straightforward roof is cheaper to build than one with corners, steps and complex junctions.
- Settle the design before you start. Changes once the walls are up are the most common cause of cost increases.
- Choose glazing carefully. Put the large openings where they matter most and use standard sizes elsewhere.
- Avoid moving services. Keep the kitchen, boiler and drains where they are if you can.
- Be realistic about finishes. Choose mid-range materials where they will not be noticed and spend on the things you will use every day.
- Get like-for-like quotes. Give each builder the same drawings and specification so you can compare properly.
- Supply some items yourself with care. Buying your own tiles or fittings can save money, but delays and wrong orders can cost more than you save, and some builders will not warrant supplied items.
- Hold a contingency. Setting money aside for the unexpected is cheaper than stopping work or compromising on quality halfway through.
What should be included in a quote
A proper quote is itemised and matches your drawings. Look for:
- [ ] Full description of the work and reference to the drawings and specification
- [ ] Groundworks, foundations and drainage, including what happens if ground conditions are poor
- [ ] Structural steelwork and engineer's details
- [ ] Roof construction, covering, insulation and any roof lights
- [ ] Windows and external doors, with the supplier or specification stated
- [ ] Electrics, heating and plumbing, and who carries out each trade
- [ ] Plastering, flooring, skirting, internal doors and decorating
- [ ] Making good to the existing house
- [ ] Scaffolding, skip hire, welfare facilities and site protection
- [ ] Building control fees, and who is responsible for arranging inspections
- [ ] Clear provisional sums for anything not yet decided, labelled as such
- [ ] VAT treatment, payment stages, start date and estimated duration
- [ ] Insurance details and any guarantee on the work
- [ ] How variations will be priced and agreed in writing
Questions to ask a builder
- Can I see examples of similar extensions you have finished, and speak to those customers?
- Who will be on site day to day, and are you using your own team or subcontractors?
- What public liability insurance do you hold?
- Who deals with building control, and who is the structural engineer?
- What is not included in this price?
- How will you handle unexpected problems in the ground?
- How will you protect the rest of the house from dust, and how will we keep living in it?
- How are payments staged, and what triggers each one?
- Is the Party Wall etc. Act likely to apply, and who will deal with the neighbours' notices?
- What happens if the programme slips?
How long it takes and what to expect
A single storey extension usually takes a few months on site, not counting design, approvals and the wait for a builder's start date. Those earlier stages can take as long as the build itself.
The early weeks are the noisiest and messiest: digging, concrete and bricklaying. Once the roof is on and the building is weathertight, the pace feels slower even though a great deal is happening inside, with first fix, plastering and drying time before the finishing trades come in. Plaster needs time to dry before decorating, and rushing it causes problems later.
Expect some disruption to water, power and heating when the new work connects to the old, and a period when the opening into the house is sealed off. Building control will inspect at key stages, such as foundations, drains and insulation, so work must be left uncovered until they have seen it. At the end, make sure you receive the completion certificate and any electrical and gas certificates.
Signs a quote is too cheap or too expensive
Too cheap
- It is far below the others and the builder cannot explain why.
- The description is vague, with no itemisation or specification.
- Major items such as drainage, steelwork, scaffolding or building control fees are missing.
- Large provisional sums stand in for work that should be priced.
- The builder asks for a big payment upfront or pushes for cash with no paperwork.
- There is no insurance, no references and no fixed address.
A low price often returns later as extras, or as corners cut where you cannot see them.
Too expensive
- Allowances for groundwork or contingency are large with no reasoning.
- Items are listed as one lump sum so you cannot see what you are paying for.
- The specification is higher than you asked for.
- The builder seems unwilling to discuss changes that would reduce the price.
If one quote is out of line with the rest, ask for a breakdown and compare it against the prices on this page. A good builder will be happy to explain the figure.