Why there's no single price
Online averages for extensions cover everything from a small single-storey room to a large kitchen extension with a glazed roof, so they are rarely much help for planning a real project. What you pay depends on the size and shape of the space, what is happening in the ground, how the roof is built, what goes inside the room and the standard of finish you want. Each of these is covered below.
Size and shape
Floor area is the starting point, but shape counts as well. A simple rectangle off the back of the house uses materials efficiently. Wrap-around or L-shaped extensions, angled walls and changes in floor level all add complexity to the walls, roof and junctions with the existing house. Opening up the back wall to connect old and new space usually needs structural steel or a beam, which is its own line on the quote.
Groundwork and foundations
What lies under your garden can shift the budget more than almost anything else. Soil type, nearby trees, drains running beneath the footprint and the depth needed to reach firm ground all affect how much digging and concrete the foundations need. A sewer or drain under the planned extension may need diverting or protecting. These factors are often only clear once the site has been looked at properly.
Roof type
Flat roofs are simpler to frame and are common on single-storey rear extensions. Pitched roofs need more timber and tiles, and the way they meet the existing house takes careful detailing, but they often suit the look of older homes. Roof lanterns, skylights or a partly glazed roof bring in light and add to both materials and labour.
Doors, windows and glazing
Large sliding or bi-fold doors and big picture windows transform how an extension feels, and they are often one of the larger single items. The size of the openings affects the structure above them, and higher-specification glass for heat retention or security adds cost. Matching new windows to the rest of the house can also influence the choice.
Kitchens, bathrooms and services
A room that needs water, waste, extra electrics and ventilation costs more to build than an empty living space. Kitchens and bathrooms bring plumbing runs, more sockets, extraction and often new flooring and tiling. The fitted units and appliances themselves vary enormously in price and are sometimes supplied separately, so it is worth being clear what a quote includes.
Drawings, planning and building regulations
Most extensions need some form of drawings, and many need structural calculations. Some can be built under permitted development, while others need a planning application, depending on size, position and the property itself. Almost all extensions need to meet Building Regulations, which are checked during the build. The rules vary from home to home, so check with your local council early on, before plans are settled.
Finishes and what's in the quote
Two extensions with identical shells can differ widely once the finishes go in. Flooring, plastering, decorating, lighting, heating and skirting are all part of the final cost. When comparing quotes, check whether each one covers the shell only or a finished room ready to use, and whether items such as kitchens, flooring and decorating are included.
How Daniel prices a project
Daniel starts by visiting the property to look at the ground, levels and access, and to understand how the new space needs to work alongside the existing house. He then prepares a quote based on what he has seen. Extensions sit alongside landscaping in the business, so the garden around the new room can be planned at the same time, whether that means a new patio outside the doors or a fuller garden renovation.
Read more on our extensions and renovations page, or call 07841 510110 or use the contact page to arrange a visit.
Frequently asked questions
Why do extension quotes vary so much?
Quotes often cover different scopes. One may price the shell only while another includes finishes, kitchens or glazing. Groundwork assumptions and roof type can also differ, so compare what each quote actually includes.
Does a flat roof or pitched roof change the cost?
Yes. A flat roof is usually simpler to build, while a pitched roof needs more timber, tiles and careful detailing where it meets the house. Roof lights or glazing add further cost to either.
Is a kitchen extension more expensive than a living-room extension?
Usually, because a kitchen needs plumbing, drainage, extra electrics, extraction and fitted units. A living space without those services is generally simpler to complete.
Do I need planning permission for an extension?
Some extensions can be built under permitted development and others need a planning application. It depends on the size, position and your property, so check with your local council before committing to a design.
What should an extension quote include?
Look for groundwork and foundations, structure, roof, doors and windows, services, plastering and any finishes, plus who is responsible for drawings and approvals. Anything not listed should be clarified before work starts.