When considering a house extension there are a number of different construction methods available to you, including timber frame extensions.
This type of extension is often quicker and easier to build than blockwork, and in its most basic stick built form, can be a great option if you're building on a budget.
However, with sub-categories falling under the general timber frame umbrella, there's plenty to consider, not least of all which type you choose and how it will look when finished. Self-build experts, Mike Cruickshank and Mark Stevenson, explain what you need to think about when choosing a timber frame extension.
What are the different types of timber frame extensions?
"Timber build systems for extensions tend to fall into three main categories: open panel, closed panel, and structural insulated panels (SIPs)," explains build expert, Mark Stevenson.
"Open panel systems tend to be assembled on site as 'stick built' and offer flexibility. Closed panel systems are usually manufactured in factories to predetermined designs and are pre-insulated with services zones, which speeds up the build programme," he explains.
Oak frame extensions are of course also included under the umbrella of a timber frame extension, and often come in at the top end of cost and design.
"SIPs take things a step further too," says Mark, "combining structure and insulation in a single component. SIPs also provide reduced thermal bridging with specialist jointing systems, making them particularly attractive where airtightness and thermal performance are essential," he notes.
If you are using a stick build process however, you will need a structural engineer to design the timber frame and prepare a specification of the size/grade of timbers to be used for the frame/lintels/cripple studs and so on, together with a nailing schedule.
A joiner will then take delivery of all the timbers required, and as they are loose, they have the advantage of being easily transportable through restricted access areas – a common occurrence when extending a terrace house for example.
The timber frame panels are then made on site following the structural engineer’s specifications, and because the existing structure will be be opened up/exposed at this point, it allows him to take accurate dimensions and manufacture the panels to suit.
If your chosen design features large expanses of architectural glazing, then a steel or glulam portal frame/goal post arrangement may also be required. This can be fabricated off-site, delivered in sections and bolted together on site.
Mark Stevenson is a construction professional with almost 35 years’ experience across housebuilding, timber systems manufacturing and bespoke residential development. Previously Managing Director of Potton and former Chair of NaCSBA and the Structural Timber Association, Mark is widely recognised for his specialist knowledge of timber construction, land finding and appraisal, and the construction of self-build projects.
What are the pros and cons of timber frame extensions?
There are many benefits to building a timber frame extension, and not just the extra space and added value that it gives to your home.
With a timber frame extension, you can achieve the weathertight stage in days rather than weeks, which not only means less time spent on site, but also less dependence on good weather conditions and reduced levels of disruption.
Once built, a timber frame extension can also offer good levels of thermal performance and airtightness, making it suitable for projects with sustainability at their core.
"The biggest advantage of a timber frame construction is speed and predictability," agrees Mark Stevenson. "Once the frame is manufactured, it can be erected in a matter of days, which dramatically reduces construction time on site and exposure to weather.
"On the downside," he notes, "it requires a higher level of upfront design coordination – everything from window openings to service runs needs to be resolved early. This often means that prefabricated timber frames and SIPs have longer lead in times. There’s also less tolerance for last-minute changes compared to traditional masonry."
"Timber frame really comes into its own on projects where speed, disruption and buildability are key considerations," he continues. "For example, tight sites, where neighbours are close by, will benefit from the reduced construction times, with less disruption from noise and dust.
"That said, it’s not always the best option where fire is a concern and fire mitigations are needed to prevent fire spread during construction," he warns. "Sites with extremely restricted access for delivery or lifting equipment can also make closed panel timber building systems impractical."
It's also important to note that any part of the extension that is below finished ground level means a timber frame construction would not be suitable for that element of the build, meaning basement conversions are not suitable for timber frames.
Timber frame extensions rules and regulations
In much the same way as other types of extensions, timber frame extensions fall within the general guidance around planning permission for extensions.
If your proposed extension meets certain criteria in terms of size and position, then you may be able to extend under Permitted Development Rights (PD). But, if you want to build something outside of the scope of PD, then you will need planning permission.
Always double check with your local authority before starting work, especially if your property is listed or located in a conservation area or National Landscape (formerly known as Areas of Outstanding Natural Beauty).
Your timber frame extension will also need to adhere to the current building regulations and the relevant approved documents.
Costs of building a timber frame extension
In terms of costs for a timber frame extension, as with other extension costs, there are a number of factors at play. Size, access, finishes and how it will connect to your existing home will all play a part, as will the type of timber frame you choose as your construction method.
Homebuilding & Renovating's content director, Michael Holmes recommends budgeting the following amounts if you are considering a timber frame extension:
- Stick built: £1,900 - £2,400 per sqm
- Factory manufactured: £2,200 - £2,800 per sqm
- SIPs: £2,400 - £3,000
- Oak frame: £3,500 - £4,500
Also, as with any home improvement or extension project, remember to weigh up whether it will add more value to your house than it costs to undertake the work.
Exterior finishes for timber frame extensions
While the interior of your timber frame extension can be finished much like any other extension, you will also need to think about external cladding and how you want the exterior of your timber frame extension to look.
While oak frame extension ideas often continue the oak theme, SIPs or stick and build systems can be finished with any of the following cladding ideas:
- Facing brick
- Rendered brickwork/blockwork
- Natural stone
- Reconstituted stone
- Brick slips
- Timber cladding
- Fibre cement cladding
- Metal cladding
- Render board system
Worth noting is that if your timber frame extension is being built on a boundary then the external finish has to be non-combustible and provide a minimum one hour’s fire resistance to satisfy building regulations.
Tips for building a timber frame extension
Where any extension (irrespective of the construction method used) is attached to the existing building it is very important to ensure a movement gap is formed to accommodate shrinkage, thermal and moisture movement.
It's usually the case that a timber frame extension uses a different construction method to that of the existing building, and as such will expand/contract at a different rate to the existing building, hence the requirement for a movement joint.
Movement joints are typically formed by using a stainless-steel channel tie system which provides lateral restraint but allows for both horizontal and vertical movement. The gap formed between the two structures is filled with a flexible sealant (different colours are available) to make it weathertight.
In addition, make sure you bear the following in mind when opting for a timber frame extension:
- Choose a designer with experience in designing timber frame extensions
- Get three detailed quotes and go over them with your designer to ensure you are comparing on a like for like basis
- Always have a contingency budget of around 10% to deal with any unforeseen circumstances
- Don’t forget to budget for external works including paving, decking and landscaping
Mark Stevenson's final advice for timber frame extensions? "You don’t have to match the structural system of the existing house but in my opinion, it’s a good idea to avoid insurance and mortgagability complications."
However, with older properties, this isn't always an option, so "if you do opt for a timber frame extension on say a masonry house, it’s essential that the two elements are detailed and connected properly, particularly at junctions to manage structural movement," he says.
"From a planning and aesthetic perspective, however, matching external materials is often more important than matching structure. A timber frame extension can be clad in brick, render, or timber, so visually it can sit comfortably alongside almost any existing property, making it an option for most homeowners."
For more inspiration on building with timber, take a look at these timber frame extension ideas, and how this 1970s terrace home was transformed with a timber frame extension.
