FACTORS YOU NEED TO CONSIDER
The first factor to consider is the makeup of your existing building. This is going to determine whether you are better off with an off-site or on-site construction method. Newer homes are generally better suited to an off-site method such as Structural Insulated Panels (SIPs) due to the required level of precision. Significantly older homes may require a more traditional method of construction such as standard masonry.
That being said, there are a few ways to use an off-site method regardless of your buildings age. A good place to start is with an existing building survey from a RICS surveyor. This will help you determine the surveyed location, thickness and squareness of the existing walls and can help the manufacturer accurately design and set out your kit. If you opt for an off-site manufacturer, we recommend they visit to your existing property for a greater understanding of the project constraints.
We’ve written extensively about off-site methods performing better from a thermal and an airtightness standpoint, but this doesn’t mean you cannot achieve a well performing extension with more traditional means. Investing in the right builders and keeping an eye on the quality of workmanship is a must. Your chosen construction method does not have to match with the one used in the home, but we would recommend having input from a structural engineer to aid your decision.
Off-site methods may have a quicker time on site but come with access requirements which can rule them out for a lot of self-builders, especially for extensions. Assess the surroundings of your home and figure out the best way to get materials to site. If you live in a well-built-up area, getting a prefabricated kit to your site can have significant limitations.
Your requirements for energy consumption will also determine what method is best for you. Depending upon the scale of the extension and how much of the existing envelope will effectively be retained, your new walls’ specification will alter your existing airtightness and thermal envelope differently. Although it’s not a legal requirement to get a new EPC rating once your extension is completed, you can have a compensated SAP calculation conducted on completion if the performance of your home is your top priority.
CONSULT AN ENGINEER
Extending your home takes more than an architect. A structural engineer should be involved at an early stage. An engineer can advise you on the structural implications of each construction method for your extension. If you involve a structural engineer at the start of your project, this can help you save on the overall cost of your build and help make the process easy for your builder by considering buildability & reviewing the architectural proposals.
Want to learn more about a structural engineer’s role in an extension project? Click here.
WHAT ARE MY OPTIONS?
Timber Frame is a popular choice for self-builders due to its energy-efficient properties and its quick build time. The frame acts as a superstructure, meaning it supports your extension entirely. A drawback of a prefabricated Timber kit (and most off-site methods) is the access requirements to get the kit to site. The kit is heavy and bulky, and transporting it can be a challenge.
Check out our article with Fleming Homes for more information on Timber Frame.
Stick-Built (also known as on-site Timber frame) is a more economical and sustainable solution to extending your property. Unlike its off-site counterpart, the frame is made up by the joiner on site. A stick-built extension offers good levels of airtightness and thermal performance once built, but does come with an added reliance on workmanship.
Brick and Block consists of a traditional-looking external leaf of brick (or rendered block), with an inner leaf of concrete block. The cavity between the two layers is then filled with your preferred insulation. Although popularity in masonry work is decreasing for new builds, it still stands as a solid choice for extending, especially in populated areas.
Check out our article on Brick and Block for more information.
SIPS are made up of two sandwich panels consisting of two layers of Oriented Strand Board (OSB), which are then filled with rigid insulation. The end product is a panel that is both robust and structurally sound. Much like a Timber kit, SIPs come with their own access requirements, including the use of a crane to install the roof.
Check out our page on building with SIPs for more information.
ICF consists of hollow polystyrene blocks, interlocking together to create a mould into which ready-mixed concrete is poured. This super-fast build method allows more adventurous designs to be achieved, whilst tackling the insulation at the same time. ICF still possesses access requirements as an off-site method, but the blockwork is largely done on-site, making transportation to site easier.
Check out our article with Econekt for more information on ICF.
Modular extensions are also available to those looking for a quick and easy solution to their extension. This off-the-shelf style method is relatively less disruptive than other build methods and can be designed to match the existing building. However, your flexibility in design is heavily restricted, so we would only recommend this for smaller, simpler extensions.
When it comes to choosing a construction method for an extension, you must take a combination of the site and your requirements into consideration. Certain build methods come with their own advantages and drawbacks, so take time to do your research and speak to the right people.
The key is to balance your needs and constraints to achieve the best possible outcome for your home. Construction costs have gone up recently, and a lot of people who are opting for extensions as a way of upsizing. An extension is always going to be more complicated and more costly than a new build house, but by being flexible and realistic about what can be achieved, you can make the process much easier for everyone involved.