Timber frame extension to a cottage by architecture practice Pensaer. Photo by Jerry Florez.
Building a timber frame extension can be a quick and sustainable way to add space to your home. There’s also potential to save on build costs by using this method.
However, there’s lots of things to consider before choosing this method, from construction options, fire protection and cladding.
Find out more in our expert guide.
In the UK, ‘timber frame construction’ generally refers to the ‘stick build’ method, where structural timber panels are made from sections of timber and sheathed in plywood or OSB for strength and stability.
Other forms include SIPs (Structural Insulated Panels), which are sandwich panels consisting of an insulating foam core, typically expanded polystyrene (EPS) or polyurethane, between two layers of oriented strand board (OSB), and CLT (Cross Laminated Timber), both cut to size off site and assembled on site.
A rear extension by Emil Eve Architects incorporating an exposed structural timber frame. Photo by Building Narratives.
Timber frame extensions can be quicker if you fully exploit dry construction methods, minimising the use of concrete and wet screeds. However, many projects still require new groundworks and steelwork, limiting the overall time savings. Timber’s lighter weight compared to masonry allows for smaller foundations, and using Glulam beams and clever engineering can reduce the need for steelwork, helping to shorten the build time.
Additionally, pre-manufactured timber frames can speed up construction if lead-in times are managed effectively.
Timber construction is more sustainable than traditional methods, particularly if you reduce the use of concrete and steel, which are significant sources of carbon emissions. Timber’s lighter weight allows for smaller foundations, and installing a suspended timber floor and timber frame walls instead of concrete and brick can significantly lower your project’s carbon footprint.
Using sustainably sourced timber certified by the Forest Stewardship Council (FSC) or Programme for the Endorsement of Forest Certification (PEFC), along with wood-based insulation, interior linings, and cladding, further enhances sustainability.
A Victorian house extension by Cairn, constructed using a hardwood frame and hempcrete, minimises the use of steel, concrete and plasterboard, achieving a lifetime carbon reduction of 40% compared to a conventionally built structure. Photo by James Retief.
In the UK, blockwork cavity walls are generally the most cost-effective construction method. Concrete blocks are inexpensive, and labour is readily available. However, if the extension’s floor and roof are timber, completing the walls in timber frame may be more cost-effective since the necessary labour is already on-site.
Cavity blockwork walls generally range from £800 to £1,000 per square metre, while ‘stick build’ timber frame walls tend to cost between £1,000 and £1,200 per square metre, depending on the specification and finish.
SIPs and CLT, while offering faster construction, are typically more expensive than other methods.
Timber I-joists, though more expensive, are more stable and less prone to twisting or creaking compared to traditional timber sections. They offer greater precision, as cut timber can sometimes be uneven, and they are lighter, making them easier to transport and handle. I-joists are also suitable for running services through and can come with integral insulation, helping to reduce thermal bridging.
Off-site manufacturing offers speed and quality but requires good access to transport large panels. Coordination is crucial to ensure correct installation. If the connection between the extension and the existing house is complex, off-site manufacturing may have to wait until the build is well advanced, delaying progress.
Stick-built frames are usually assembled on-site but can be built off-site for improved quality. SIPs and CLT are always manufactured off-site and assembled on-site.
A prefabricated Douglas fir extension by architect Christian Brailey. Shown under construction in the workshop (left), craned into position (right), and as the completed project (bottom).
Timber frames can be clad in a variety of materials, such as:
Cladding in masonry is common, and sometimes required, but finishing with timber or lightweight materials can be more architecturally appropriate and speed up construction.
Timber frame construction can be highly fire-resistant with the right materials: fire-retardant linings (plasterboard or fire-resistant panelling), mineral wool or wood fibre insulation, and masonry or cement cladding. If the extension is near a boundary, the external cladding must be non-combustible and provide at least one hour’s fire resistance to meet Building Regulations.
Timber frame extension by Pensaer. Photo by Jerry Florez.
Timber frames are prone to interstitial condensation if inadequate vapour protection is installed, which can lead to rot and mould. A combination of breathable insulation and a high-quality vapour control layer is recommended to manage moisture. A breather membrane should also be installed to prevent water ingress while allowing water vapour to escape. Raising the floor above ground level is essential to avoid water ingress and ensure ventilation.
If you're funding your extension with a mortgage, check for lender restrictions on timber frame construction and cladding materials. If the main house is masonry and the extension is certified by a structural engineer, some lenders will approve a timber frame extension. However, timber cladding may be more difficult to finance – many lenders prefer brick or stone cladding, as these are considered ‘standard’ non-combustible materials.
Always confirm with your lender before starting the project to avoid issues later.
A brick-clad extension by Nimtim Architects. Photo by Megan Taylor.
One of the advantages of timber frame construction is its ease of thermal efficiency. High levels of insulation and airtightness can reduce heating demand, meaning smaller radiators or heating systems may suffice.
Underfloor heating can be used with timber, though it is less effective than with concrete due to timber’s lower thermal mass. However, with proper insulation, the extension may not require significant heating at all. In some cases, small fan heaters or a wood-burning stove could be enough, depending on how open the extension is to the existing house.
READ: How To Insulate Your Home
By following these steps, you’ll be well-prepared to build a timber frame extension that is cost-effective, sustainable, and well-suited to your home.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
