How to Build Your Extension: Structural Options

Looking for an efficient, value-for-money system to ensure your new addition delivers a comfortable living environment all year round? Here’s what you need to know
Articles by Build It magazine
by Build It
8th December 2020

Deciding exactly how to build your extension should be a three-pronged conversation between you, the architect or designer and your on-site construction team. The goal is to select the system(s) that best fit your budget and project profile. This guide will introduce you to the main options, and some of the key considerations that might tip you towards one solution over another.

Walling Systems

You can extend a house with pretty much any construction method, but it’s fair to say standard cavity wall blockwork remains the most popular option for conventional designs. That’s largely because it’s a familiar and affordable choice for adding space to a brick-built home (which accounts for the majority of the UK’s older housing stock).

The exact specification will depend on your project, but a typical cavity wall that meets Building Regs would consist of 100mm-thick internal blockwork, a 100mm gap that gets fully filled with insulation, and an external leaf of facing bricks (or blocks to carry render, timber cladding or another finish).

Masonry isn’t the only option, of course, and plenty of homeowners go for options such as timber frame (usually prefabricated, but stick-build may suit difficult-to-access sites) and structural insulated panels (SIPs). Both can deliver equivalent insulation values in a thinner wall profile than blockwork, so you may be able to eke out a little extra internal floor space with these systems.

Basement Extensions: The basics

If you’re planning to excavate a new basement living space beneath your house (or your cellar conversion project involves lowering the existing floor), the foundations will need to be underpinned to the required depth. This is a major engineering operation, and the works must be properly designed and sequenced by a professional to ensure the safety of your home and the contractors undertaking the project.

Fundamentally, this is done by splitting the process up into manageable sections. The crew digs under one short area of wall, preps it and installs the new footings (usually by pumping in concrete), before moving on to the next section. The level of work involved makes this the most time consuming and costly part of a basement extension.

Another critical area is the waterproofing, which should be designed and installed by a specialist. For this reason, it’s best to involve this expert in your scheme right from the early planning stages.

There are three main types of waterproofing for the walls and floors of basement extensions: Type A barrier systems (such as spray-applied slurries); Type B structurally integral methods (such as waterproof concrete); and Type C cavity drainage membranes (CDMs) that use an egg crate-style membrane to direct water into perimeter drains and then pump it out.

In most cases, two waterproofing systems will be specified. In many cases this will be a combination of Type C with Type A; although the exact design will depend on site conditions. Many insurers require this dual-protection approach, and will insist that you use a specialist who offers an insurance-backed guarantee on their installations.

Learn  more: Basements – What can go wrong?

When completing this kind of work, always use contractors who can demonstrate proven track records of completing successful basement projects. Not every general builder has the experience and knowledge to deliver a high-quality, properly waterproofed space – including critical details such as managing health & safety and proper protection of the waterproofing systems while the rest of the works are ongoing.

The factory-made panelised systems are super quick to build with, so the shell of your
new extension can be made weathertight in just a few days. They’re also lightweight structures, which can be an advantage if you’re building on top of a garage or an existing single storey zone of your home – minimising the amount of additional support required to enable the extension.

Green oak framing is a popular option among homeowners who want to create a space that’s packed with character. It’s another quick system and fantastic for creating dramatic features such as vaulted ceilings (as are SIPs). Insulation is usually provided in the form of encapsulating timber or SIPs panels, which will form the substrate for your cladding. Most oak extensions are completed on a design and build basis by a specialist company such as Oakwrights and Welsh Oak Frame.

There are various other solutions that might better suit your project profile. Insulated concrete formwork (ICF) is a LEGO-style system where hollow polystyrene blocks are stacked and filled with concrete. This is another speedy system – and will give you the same sense of solidity you’d get with a masonry extension.

Learn more: Build Systems – Insulated Concrete Formwork

If sustainability is of critical importance to you, then you might consider a natural construction method such as hempcrete or straw bale – but bear in mind fewer contractors know how to use them. Meanwhile, if you want a modern, high-spec glass box addition, then you’ll need to go to a firm that specialises in frameless glazed structures.

Modular extension

Moss Design & Build fabricates modular builds crafted from timber components. Its extensions are manufactured in the company’s workshop before being assembled by a team of skilled joiners on site

The new kid on the block is modular extensions. This is where whole pods or rooms are built in the factory, with internal and external finishes already applied, and craned into position on site. While it won’t come cheap, prices are becoming more competitive, and with this method a single storey extension might be ready to move into in as little as a week. It’s suitable for loft conversions, too. The technique can work particularly well in city-centre locations, where getting in and out quickly and on a cost-certain basis may be attractive.

Loft conversions: Structural considerations

Achieving sufficient head height If there’s at least 2.3m of space between the top of the attic floor joists and the underside of the ridge beam, then you’ll usually be able to create a habitable zone with a straightforward conversion project. If not, you may need to replace the existing roof structure or lower the floor level, which will add cost.

Dealing with trussed roofs If your home was built after about 1960, it probably features W-shaped Fink trusses in the roof. These will feature lots of bracing, designed to minimise the thickness of timber required. Reconfiguring them to create a clear loft void is very much achievable, but you’ll need to involve a structural engineer.

Insulating at roof level To create a comfortable loft room, you’ll need to add thermal protection in the roof. The most affordable way to do this is to insulate between and beneath the rafters – but this approach could encroach on available headroom.

Learn more: Loft Conversion Projects – A complete guide

The alternative route, which involves lifting the roof covering and insulating above the rafters, will provide a completely insulating wraparound for better thermal performance, but is likely to increase the height of the building (you won’t usually need planning permission unless you’re adding more than 150mm). The extra cost of this method means it is typically only viable if you need to replace the roof finishes anyway.

Repairing defects You might be put off a loft conversion project if you spot rotting timbers, defective roof tiles or undersized joists in the roof void – but these are common issues that a good builder or conversion specialist will be used to dealing with.

Beefing up the floor Most loft floors aren’t floor structures at all – they’re designed simply to stop the rafters spreading and to carry the finishes of the ceiling below. So thicker joists and elements of steelwork may need to be inserted (which could impact on finished head height in your liveable loft). You’ll also want to consider upgrades such as fitting sound insulation into the floor.

Joining the extension to the house

Every new structure takes time to settle into position, so there will always be an element of differential movement between the extension and the existing house – even if you’ve used exactly the same materials. To accommodate this, a movement gap will be formed where the two join, which basically serves to allow the new addition to move independently of the main building.

Foundations for Extensions

The first thing to get right on any type of extension project is the foundations.

To do this, your project team needs to understand what’s supporting the existing house; identify the soil conditions on site; and investigate the potential for any nearby trees, services and drains to impact on the new foundations. All of these considerations can have a major impact on the foundation design.


By and large, existing houses have relatively shallow footings. Most new homes and extensions, however, are built with foundations excavated to a depth of 1m or more. This change came about after the long, hot summer of 1976, when a significant flurry of insurance claims for subsidence led to an update to the Building Regulations. The discrepancy between modern requirements and the existing, older foundations can lead to problems with differential movement between the two structures. To get around this, in some cases the new foundations will be gradually stepped away from the existing building.

If your planned extension is close to a boundary or within 6m of your neighbour’s property, you may need to come to an agreement under the Party Wall Act. This must be done prior to excavating any trenches for your foundations. Contrary to what you might have heard, the Act isn’t there to give your neighbour a chance to prevent the project from going ahead: it simply provides a framework for preventing and resolving disputes.


The easiest type of foundation to build is an excavated trench which is then mass filled with concrete (hence the name, trench fill). The concrete is usually taken all the way up to 75mm below the top of the excavated trench walls, and then a course of blockwork is installed up to damp proof course (DPC) level. This gives you a standard minimum clearance of 150mm above external ground level for the superstructure above (subject to the detailed design for your project).

Learn more: Foundation Solutions for Home Extensions

Strip footings work according to the same principles, but the depth of concrete is restricted to the bottom of the trench only, laid up to a thickness of around 200-400mm (usually with a couple of reinforcement bars). The rest of the foundation wall is constructed in dense concrete blockwork, directly under the above-ground external walls, with a DPC separation at floor level.

Concrete costs mean trench fill is a bit more expensive, but most people go for
it as it’s so quick and easy to install. Widths are usually 450mm or 600mm to match excavator bucket sizes. The external wall system, which is typically around 300mm thick, then sits directly in the middle of the foundation. The trench sides should be vertical, so you can be sure the concrete is substantial all the way to the base.

It’s not unheard of to dig down and find something unexpected. If this occurs, you will need to switch to engineered foundations, such as a concrete raft or piles driven into the ground. This will add a significant chunk to your project budget, so it’s a wise move to set aside a good contingency fund for this phase of the works.

The most common way to do this is using a wall starter kit, which is basically a special bracketing system that allows for a little movement both vertically and horizontally. The small gap between the extension and the main house is then filled with a flexible, long-lasting jointing compound.

One variant you might see on masonry builds is toothing out alternating courses of the existing wall, so the new facing bricks can interlock. Some builders prefer the finish this gives (as opposed to having a straight cut line all the way up). The internal blockwork leaf will still be connected with starters.

If you’re extending to create a large open-plan living area, a steel beam will probably be needed to span the zone where the existing wall is to be removed. This will support the wall and roof loads above.

Ideally, this will be located within the floor structure – but sometimes the cost is prohibitive and it makes more sense to drop it below the ceiling and accept that there will be a boxed-out section. Either way, the steel will be inserted before the wall is removed. A structural engineer should determine the size of the beam and installation method.

Roof structures

The style of your extension’s roof is likely to be influenced by the look of the house, location of windows and any planning constraints. Pitched designs can tie in well with the rest of the architecture, particularly if you opt for the same tiles. Flat roofs, which are actually formed at a very slight angle to allow for rainwater runoff, tend to make a contemporary statement.

If there’s enough headroom in your attic, then a simple rooflight conversion could be the most cost-effective way to add space to your home

Either way, on small extensions the roof structure will be cut and built on site – rather than using prefab elements such as trusses. Extra timbers will be added to form the structural openings for elements such as rooflights and lanterns. Many people opt to insulate at rafter level, creating a warm roof with vaulted ceilings that maximises the sense of space.

If you’re concerned about the potential for flat roofs to leak, the good news is there’s no need to be. Modern materials such as single-ply membranes now offer excellent long-term performance. You can also consider alternatives such as a green or living roof. These are typically specified as a lightweight sedum system, which means you can usually still specify a conventional flat roof structure (without beefing it up).

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