If you are pricing up a single storey brick extension in South Wales, most projects currently land somewhere between £1,800 and £2,500 per square metre for a straightforward build. That is a wide range, and where your project falls within it depends on size, ground conditions, the finish you want and how much of the work involves opening up the existing house. This guide breaks down realistic figures for the Cynon Valley and wider South Wales area so you can budget with your eyes open.
South Wales build costs sit below London and the South East, but they have risen noticeably since 2021 because of material and labour prices. For a single storey brick extension to a basic but decent standard, expect roughly £1,800 to £2,100 per square metre. A mid range specification with better glazing, underfloor heating or a higher standard of kitchen fit out typically runs £2,100 to £2,500 per square metre, and high end finishes can push beyond that.
To put that in real terms, a modest 15 square metre extension might cost £28,000 to £36,000, while a larger 25 to 30 square metre kitchen extension often lands between £48,000 and £70,000 once fitted out. These figures usually exclude VAT, professional fees and the kitchen or bathroom units themselves, so always check what a quote actually covers.
Two extensions of identical size can differ in cost by tens of thousands of pounds. The biggest single factor is usually what happens where the extension meets the house. Knocking through a structural wall to create one open plan space needs steel beams, structural calculations and temporary supports, which adds cost quickly compared with keeping the new room separate with a standard doorway.
Ground conditions matter a great deal in the Valleys. Sloping plots, made up ground, clay, old mine workings and nearby trees can all mean deeper or engineered foundations. On some Cynon Valley sites a coal mining risk assessment is needed before building control will sign off the foundations, and drains running under the proposed footprint may need a build over agreement with Welsh Water.
On a typical single storey extension, roughly a quarter to a third of the budget goes below ground and into the shell: excavation, foundations, drainage, blockwork and facing brickwork. The roof, windows and doors take another significant slice, and the remainder goes on first and second fix trades such as electrics, plumbing, plastering, flooring and decoration.
Brickwork itself is rarely the expensive part. A competent bricklaying team laying facing brick in a standard stretcher bond is good value for what it delivers, which is a structure that should outlast most of the alternatives. Where costs creep is in everything around the brickwork: scaffolding, waste removal, skips, and the finishing trades at the end of the job.
Many single storey rear extensions in Wales fall under permitted development, meaning no planning application is needed, provided they stay within limits on depth, height and how much of the garden they cover. Wales has its own permitted development rules that differ slightly from England, so do not rely on English guidance. If your house is in a conservation area, or the extension goes beyond the limits, you will need a planning application to Rhondda Cynon Taf council, which currently costs a few hundred pounds plus drawings.
Building regulations approval is required either way. Budget around £600 to £1,000 for building control fees and £1,000 to £2,500 for architectural drawings and any structural engineer input. If you share a boundary wall with a neighbour, party wall notices may also apply before work starts.
Get at least three written quotes and make sure each one itemises what is included: foundations, drainage, roof covering, windows, internal finishes and VAT. A quote that is thousands cheaper than the others has usually left something out rather than found a clever saving. Ask who is responsible for building control inspections and whether the price includes making good where the extension joins the house.
Be realistic about timescales too. A typical single storey extension takes 10 to 14 weeks on site, longer if steelwork, bad weather or drainage surprises get involved. A sensible contingency is 10 to 15 percent of the build cost, and on older Valleys terraces where you never quite know what is behind the render, leaning towards 15 percent is wise.
A small extension of around 12 to 15 square metres typically costs £25,000 to £36,000 in South Wales, depending on ground conditions and finish. That usually excludes kitchen units, professional fees and VAT, so confirm exactly what any quote includes.
Often not, as many rear extensions fall within Welsh permitted development rules on size and height. You will still need building regulations approval, and if you are in a conservation area or exceed the limits, a planning application to your local council is required.
A brick and block extension usually costs slightly more upfront than a timber frame of the same size, but the gap is smaller than many people expect. Brick offers excellent durability and low maintenance, and it tends to blend better with existing South Wales housing stock, which helps resale value.
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