Basement conversion guide

With floorspace at a premium in many UK houses, basement conversions will give you extra room and can add around 10% to 15% to the value of your home. Here’s all you need to know before transforming your subterranean space.

According to Harvey Norman Architects, “A basement is a storey whose floor level is more than two metres below ground level.” And with floorspace at a premium in many UK houses, it’s unsurprising that there’s been a sharp increase in basement conversions in recent years. Not only will they give you an extra room, but they’ll add considerable value to your property; Zoopla estimates a basement conversion will add “around 10% to 15% to the value of your home”. And this estimation can go up urban areas, where there’s no option to extend and limited outdoor space, making adding a subterranean level particularly desirable. The advance in technology in glazing solutions has also contributed to a rise in basement conversions, as they’re no longer dark underground spaces but can be architectural gems in a property.

Depending on the type of property and design plan you have, you may be looking at a basement conversion, extension or excavation. A basement conversion involves transforming an existing basement space, and not changing the home’s existing footprint. This is common in Victorian, Edwardian, and Georgian homes, which often had basements to accommodate house staff. Period houses with suspended timber floors are particularly ideal for basement conversions.

A basement extension involves digging additional space beneath an existing structure either up to increase the height, or out to extend the footprint and foundation. A basement excavation means digging beneath the structure. A retrofit, in this case, will be the most costly option. And basements generally become more expensive the lower you go. 

Assess the feasibility

The first step in considering a basement conversion is to check if your home is suitable for one and if it will be possible. Before you get started, follow this initial checklist of considerations:

• Structural assessment—You’ll need to thoroughly check your foundations for cracks and stress and assess whether they can handle additional load. The condition of load-bearing walls is also crucial in a basement conversion. Due to age, certain types of buildings will need more structural support. Older houses with shallow foundations may need to be underpinned.
• Investigate the water table—Water is the enemy of any basement conversion, so looking into your area’s type of water table is key. A high water table can complicate your excavation, making soil drainage more difficult and you may need to install a drainage pump. Check there’s no history of flooding. You can find this information out from your local authorities. Also, check your soil type and soil stability. Chalky soil drains far better than clay-based soil, for example.
• Building access—Consider site accessibility for getting machinery onto the site and if you’ll have any complications with disruption to neighbouring properties, this is particularly tricky in urban areas.
• Ventilation—Depending on how good the natural ventilation in your basement conversion is, you may need mechanical ventilation to help manage any penetrating damp, leaking pipes, seepage, and condensation.
• Insulation—Look into what type of insulation you want for your basement conversion early in the process, the options include fibreglass batts, mineral wool, rigid foam boards, and spray foam.

Check your planning permission

The Planning Portal states, “Converting an existing residential basement into a living space is in most cases unlikely to require planning permission as long as it’s not a separate unit or unless the usage is significantly changed or a light well is added, which alters the external appearance of the property.”

However, “Excavating to create a new basement, which involves major works, a new separate unit of accommodation, and/or alters the house’s external appearance, such as adding a light well, is likely to require planning permission. If you live in a listed building, you’ll likely need consent for internal or external work. In all circumstances, you’re advised to contact your local planning authority for guidance on local policy before starting any work”. Follow this link to find your local planning authority.

Regarding party walls, Resi explains that “Two months before any work commences, you need to serve notice to all legal owners of any property affected by your proposed build. This can be a simple process if you receive an agreement within seven days. However, if permission isn’t granted, you’ll need to hire one or two party wall surveyors to assemble a party wall agreement with your neighbour, before work can proceed”.

Create your design brief

Having a clear idea of what you want to use your space for should be your first port of call for a basement conversion. Defining its purpose, be it a bedroom, workspace, bathroom or utility room or home gym, will give shape to everything from the layout to the light sources and whether any internal structural supports will impact the space.

When creating your brief, think about maximising head height. While two metres is considered the minimum, it’s worth considering your options and going up to 2.3 metres and above for it to feel more airy and spacious. Factor in the measurements of the insulation in the planning stages, as dependent on its height, it risks taking the overall height lower than regulations allow.

How to waterproof your basement conversion

Xul Architects explains, “Various construction methods are available for basement walls, including poured concrete, concrete block, masonry, and precast panels. Each has its advantages; for instance, poured concrete is generally more robust and an excellent choice for mitigating water ingress.”

Given that they’re below ground, basements are far more susceptible to water and moisture infiltrating from the surrounding soil, so waterproofing is essential to prevent seeping walls, floors, and foundations. Exposure to moisture can weaken a basement structure over time, so make sure your waterproofing is as high-quality as possible. There are several types of waterproofing available for basement conversions, including sealants and epoxy injections, interior drainage systems, or exterior waterproofing with gutters and downspouts, and they’re divided into three categories, which the Property Care Association highlights as:

• Type A – Tanking – “A continuous waterproof barrier is applied to the inside or outside of the basement structure.”
• Type B – Structurally integral protection- “Usually water-resistant concrete in combination with a waterproof membrane”.
• Type C – Drained cavity – “Cavity Drainage Membrane is applied to the internal walls and floor to collect and drain away water entering the basement.”

How to add a bathroom

Barrie Cutchie, Design Director at BC Designs, says, “Adding a bathroom to a basement can sometimes be tricky. Basements can be prone to moisture and dampness issues. Ensuring proper waterproofing is critical to prevent damage and costly future repairs, and failure to do so can lead to mould and structural problems, especially in a bathroom with more moisture.”

“This is also a reason why adequate ventilation is essential. With a lack of windows and fresh air to circulate, moisture from showers and baths can facilitate mould and mildew growth, which is why an effective ventilation system is crucial.”

“Basement bathrooms often need sophisticated plumbing solutions, like sewage ejector pumps, particularly if the basement is below the sewer line. This adds complexity and cost to the installation but is a necessity when adding a bathroom.”

Louise Ashdown, Head of Design at West One Bathrooms, adds, “A basement bathroom will only add value if the rest of the space is complete and liveable. For example, if you’re creating a guest room and en-suite and potentially some additional living space or a utility room, you’ll often recoup the money you’ve spent.”

Maximise on space

Adding height to your basement by creating the tallest ceiling possible, within structural, budget, and time constraints, will give you the most freedom and feeling of space, but there are other gains you can try. Create an open-plan layout (or broken-plan) by minimising internal walls. Add beautiful built-in cabinetry to maximise space. Wood Works Brighton is a great company for perfect made-to-measure furniture.

Think about shifting your staircase. Although there are rules to adhere to—Allcot Residential states that “The stairs from the home to the basement should have an angle of no more than 42 degrees pitch with headroom no less than two metres and suitable handrails”—you can be creative. For example, you can change the position or direction of a straight run, install a cantilevered stair, add winders to create turns or opt for a helical or space-saving spiral design.

Add a swimming pool to your basement conversion

When it comes to building a swimming pool in your basement, Simply Basements says, “The average time it takes to complete a basement conversion is 6-10 months. However, this can vary depending on the complexity of the build. For a pool, you will need to factor in room for the pool itself, a plant room for filtration, air handling, and a dehumidifier, as well as some space for relaxing.”

“Depending on the type of pool you opt for, it may need to be plumbed into the water main. Another thing to consider when choosing your pool is that going for a dug-out pool requires less heating than an above-ground pool.”

Regarding different options, you could go for a lap pool, which is excellent for small basements, and if you want to use the pool for fitness. You could also go for a pool with a retractable floor, so you could vary the height, or go for a pool with a separate hot tub. You’ll need to consider planning permission for any pool you add to your basement conversion.