Oak frame extensions are one of the most asked-about projects we see across Somerset, Devon, Wiltshire and Dorset, and one of the hardest to price from a quick online search. The honest answer is that the oak frame itself is only part of the bill, and the rest depends heavily on your site, your finishes and how much glazing you want. Here is what homeowners in the South West can realistically expect to pay, and where the money actually goes.
For a finished single storey oak frame extension in the South West, most homeowners should budget somewhere between £3,000 and £4,500 per square metre, fully built and decorated. A modest 20 square metre garden room might land around £60,000 to £85,000, while a 40 square metre kitchen and dining extension with a large glazed gable can push past £150,000 once you include the kitchen itself.
That is noticeably higher than a conventional blockwork extension, which currently sits around £2,000 to £3,000 per square metre in this region. The difference is not just the timber. Oak buildings tend to be specified with more glazing, vaulted ceilings and higher grade finishes, and those choices drive the cost as much as the frame.
Ground conditions matter more in this part of the country than many people expect. Clay soils around Taunton and parts of Wiltshire, high water tables on the Somerset Levels and sloping plots in the Devon valleys can all add thousands in foundation work before a single beam arrives. A soil investigation early on is cheap insurance.
Glazing is the other big variable. A full height glazed gable with slim steel or aluminium frames can cost £20,000 to £40,000 on its own. Structural glazing that sits directly against the oak needs careful detailing to cope with the timber moving as it dries, and that detailing costs money to get right.
Roof choice, whether you go for natural slate, clay tiles or a green oak framed roof with exposed rafters, changes the figure again. Listed buildings and conservation areas, which are common across Dorset and rural Somerset, often force the more expensive traditional materials.
Most frames are cut from green oak, meaning freshly sawn timber with a high moisture content. It is cheaper to buy and easier to work, but it shrinks and cracks as it dries over the first few years. This is normal and part of the character, though it needs to be planned for in the glazing and any plaster junctions.
Air dried oak has been seasoned for several years and moves far less. It typically adds 20 to 40 per cent to the frame cost, and it is mostly used where the frame meets large glazed panels or where a very crisp finish is wanted. For a lot of extensions a mix is sensible: green oak for the main structure and dried oak for door and window posts.
On top of the build figure you should allow roughly 10 to 15 per cent for professional fees and approvals. That covers architectural drawings, structural engineering for the frame and foundations, planning or permitted development checks, Building Regulations, and a party wall surveyor if you are close to a neighbour.
VAT at 20 per cent applies to extensions on existing homes, so make sure every quote states clearly whether it includes VAT. Also budget a contingency of at least 10 per cent. Older properties in this region regularly turn up surprises once the ground is opened or the existing wall is exposed, and it is far less stressful to have that money set aside.
The most useful thing you can do before approaching frame suppliers is to have a clear scope: floor area, glazing extent, roof material and the level of internal finish. Without that, quotes are impossible to compare and the lowest one is rarely the cheapest by completion.
An independent cost plan prepared at design stage, then a properly managed tender to two or three contractors, will usually save more than it costs. It also gives you a benchmark to hold the builder to as the project moves forward.
For many homeowners yes, because the vaulted space and exposed timber are hard to replicate any other way, and well built oak extensions tend to add strong value on period properties. If your priority is maximum floor area for the lowest spend, a conventional build is the better fit.
Allow around four to six months on site for a typical single storey extension, plus three to six months beforehand for design, approvals and frame fabrication. The oak frame itself usually goes up in a week or two.
Not always. Many single storey rear extensions fall under permitted development, but listed buildings, conservation areas and larger or side extensions will need a full application. It is worth checking with your local authority in Somerset, Devon, Wiltshire or Dorset before committing to a design.