How to add a timber-frame extension

Timber-frame extensions are quick and cost-effective to build, energy efficient, and offer a huge range of design possibilities. Here’s what you need to know…

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This oak-frame extension has added both living and sleeping space to this detached home in Staffordshire. Expect to pay from £2,400 per m2 for a similar oak-frame extension, Oakwrights

Timber is a readily available, easily adapted construction material making it ideal for building extensions. A timber-frame extension built using standard panels made from softwood filled with insulation, can be finished to look no different to one constructed from blockwork, structural insulated panels, or insulated concrete formwork, so aesthetics don’t have to be a consideration.

‘The exception to this is if the chosen timber-frame system is post and beam, in which some of the structural elements are left visible (see below) – typically upright posts, beams, ceiling joists, roof trusses and rafters,’ says experienced renovator Michael Holmes. It’s also possible to mix a post and beam structural frame with timber frame panels built around it as an airtight, insulated shell.

Living room with stove in timber frame extension

Architects Van Ellen+Sheryn turned a dilapidated barn in Dartmoor a large, modern family home, with Carpenter Oak responsible for the oak frame. A similar structure would cost from £35,000 to £40,000

What are the main types of construction?

Open-panel systems

The external sheet material is fixed to the softwood frames and the internal face left open, ready for the electrics and plumbing first fix, and then insulation to be installed before they are closed up with plasterboard on the inside.

Closed-panel systems

Similar to open-panel systems but with insulation fitted and channels and ducts already in place for the services, or, in some cases, with the services already installed. Panels are typically delivered without doors and windows installed, but some manufacturers deliver panels with fully glazed external joinery in place.

How much will it cost?

  • A timber-frame extension will cost from £1,800 per m2.
  • Oak is considered a premium building material, so an extension built with it will cost from £2,400 per m2.

Timber frames tend to be the most cost- effective way to build in Scotland, but are marginally more expensive than blockwork in England, Wales and Northern Ireland. However, it does depend on the type of extension – for instance, an additional roof storey will usually be more cost-effective in timber frame than blockwork, as much of the structure will be timber anyway. A post and beam frame with large sections of timber on show will add to construction costs.

Factors affecting the price will include:

  • Thickness of the frame
  • Type of frame
  • Type and amount of insulation
  • Degree of prefabrication
  • Choice of external cladding 
  • Amount and specification of glazing
  • Your location

Pros and cons of timber frames


Lightweight: Timber weighs relatively little, making it a good choice for an extra loft storey. Ground-level foundations will be less costly to build, too.

Goes up fast: The lightweight nature of the construction system means large modules made off site can be delivered and assembled quickly.

Practical for small sites: For a house with limited space for building work or storage, off-site manufacture of big components will be useful.

Versatile: It lets you create large spaces, galleried ceilings, and rooms with character if you choose to show off the beams. Outside, the building can be clad in different finishes to blend or contrast with the rest of the house.

Space efficient: Combining a timber framework with a lightweight wall cladding like timber, tiles or render on boarding can reduce wall widths, adding to internal floor areas.

Eco credentials: Sustainably produced timber matched with high-quality insulation and double glazing creates an energy-efficient space. The void between the timber studs can be filled with insulation, so the panels can achieve a high degree of energy efficiency in a relatively slim wall.

Time efficient: While the components are being made off site, groundworks can be completed. Insulation can be factory fitted prior to construction, too, saving more time and waste on site.


Specialist system: You’ll need to use a company with expertise in timber frames. Green oak frames shrink, so must be detailed correctly to avoid problems in the future. Some timbers, such as oak, have limitations with regards to spans, and you may need to introduce materials like steel to get the layout you want.

The company you choose may offer various levels of service, from simply providing the frame, to taking on the entire build, including project management. They may have an aftercare service, too, covering snagging and structural warranty.

Modern timber frame extension

This modern timber-frame extension, by Fraher Architects, is clad in a recycled scaffolding board timber rain-screen. It cost around £175,000

How to get the design just right

1. Consider the exterior finish

Externally, it’s easier to create an addition that contrasts in style to your current home than one that matches it. If you do opt for a seamless extension, make sure it mimics the original design elements of the house, such as the roof pitch, materials and mortar colour. Timber extensions are infilled with prefabricated panels, which can be clad in materials like timber, render and brick slips, or you might prefer to show the frame.

2. Make the most of vertical space with a vaulted ceiling

This is the perfect opportunity to show off exposed beams. Tall ceilings will make rooms feel bigger, too, an impression you can enhance further with large glazed doors to the garden, rooflights, a roof lantern or clerestory windows.

3. Show off the frame of the building

Whether with vaulted ceilings, glazed gable ends or exposed beams. Bear in mind that with a visible frame, the less wood that’s on show, the more contemporary the room will feel. You can enhance the modernity of the room by incorporating stainless steel into the build and showing it off. Think steel bolts orsteel tie rods, for example.

4. Give window placement extra thought

Create a view, enhance your privacy or screen out neighbouring houses by clever positioning of windows. Consider the heights of sills and the size and shape of the windows to provide panoramic aspects, taking into account that you may get the best views when seated.

5. Use timber in the connecting spaces

If, unlike the rest of your home, your extension’s architecture includes visible timber beams, adding elements of timber elsewhere will help tie the old and new spaces together. This might be oak flooring that runs throughout, introducing timber doors, exposing previously painted woodwork, or even choosing a few pieces of furniture in the same woody tones.

Dining room and kitchen in timber frame extension

This air-dried seasoned oak-frame addition in Oxfordshire measures approximately H5xW13.3xD6m and cost around £118,000, from Prime Oak

What is the build process like?

A timber-frame extension can be constructed on site using softwood and sheet panel systems. Panels can also be made off site in a factory or workshop, making assembly on site very rapid, allowing the shell of an extension to be made weathertight in days.

To benefit from this, all of the design decisions need to have been made at the time of ordering the frame – and it’s not straightforward to make changes on site. The more work that’s completed in the factory, the faster the frame will be to assemble and complete when it arrives.

Exterior of timber frame extension

This garden room in Staffordshire is built from hand-painted Accoya timber. ‘Accoya is becoming an increasingly popular material due to its quality, strength and versatility,’ explains Penny Whitlock, director at Montpelier Joinery. It cost around £50,000

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