In highly suburban areas, extending outwards isn't always an option, which is why homeowners often investigate a basement conversion instead.
However, the reality is, many basements have been left untouched for years, leading to problems with damp and ventilation that can make transforming a subterranean space more challenging.
That said, with the opportunity that having a prime expansion space already in situ presents, it can still be a project well worth pursuing – when you know how. We asked architects experienced in this field of work to share the key questions you need to explore if you are considering converting a basement, and here’s what they recommend.
1. What do you want to use the basement conversion for?
The first question to ask is why do you need the space a basement conversion will provide? Even if the basement already exists, the work it might require to make it habitable can be costly and time-consuming. Creating a new basement from scratch? Then this will involve substantial basement conversion costs and is one that could bust your budget entirely.
The most cost-effective use of a basement conversion is a simple utility basement with no windows, ideal for a plant room, workshop, or for storage. However, experts suggest you need to seriously think if this will be a good use of your time, and money.
"It is important to create a sustainable use for the new floor as creating a basement is one of the most expensive things you can do to your home," says Andrew Mulroy, director of Mulroy Architects.
"Building a new basement under your house and using it for storage is poor value, whereas creating new living and different spaces, more bedrooms or a gym isn't. We always make our clients present a proposal that demonstrates a sustainable use for all areas of a basement before we embark on the design, so that the value of the new rooms is clear.
"Normally, we tell clients that suggest they may use the new basement as storage to try again," he says. "It’s amazing how a believable and socially sustainable brief emerges once they focus on the cost and potential benefits of the new floor."
If your basement is in good condition, with a little investment in electrical and plumbing works, and finishes to the walls and floor, you may be able to upgrade it to make a great games room, home cinema or gym, providing there is a single habitable room and an enclosed staircase leading directly to an external door, in order to meet building regulations.
But, if your basement conversion plans include family rooms, kitchens, bedrooms, bathrooms or leisure facilities then there are a whole host of extra elements to consider, all of which will impact on your final basement design if you choose to go ahead.
Andrew founded Mulroy in 2008 after almost 20 years in practice, with the aim of creating a studio that produced exciting and socially sustainable architecture that anticipated clients’ future needs.
2. What is currently below your house?
If you want to create (or make use of) space below your home, there are three main routes:
- Include a new basement as part of a new build
- Renovating an existing basement or cellar
- Create a new basement in an existing home, either directly underneath, or by extending out into the garden, front or side of the house.
While the first will almost take care of itself as it will have been taken into account in your overall self build plans, it is the latter two options where serious time and investment may be required.
“First you will need to establish whether it is a true basement,” says John Everitt, co-founder of coombes:everitt architects. “Are all sides below ground or is it a partial basement, typically on a sloping site where at least one side is at or above the adjacent ground level? If it is, then this can make the tanking far simpler.
“Or, if the basement is going to be an extension then you will need to check if there are any services running around the building that would be affected by the development of a basement. This could be drainage, gas, electricity, telecoms or water supplies,” he explains.
"You will also need to consider what level the existing drains are too, so that the services in the basement will connect into them. If they are not deep enough it may mean that any drainage in the basement has to be a pumped solution.”
John Everitt is a RIBA certified architect and director and co-founder of coombes:everitt architects, an award-winning Cotswolds based practice dedicated to creating beautiful, carefully crafted homes that enhance the way people live. With more than 25 years’ experience as an architect, John’s area of expertise is designing contemporary sustainable homes within AONB and Greenbelt.
3. Are there any site or structural issues?
With the work to your basement impacting not only the house above, but also the garden and land around your home, and potentially even your neighbours' property and land, understanding the potential constraints of structural or site issues is a key consideration when converting a basement.
“You will need to work closely with architects, engineers and contractors that know what they are doing so that the structural stability of the existing and neighbouring houses are not affected,” confirms Andrew Mulroy.
“The depth of excavation will be a major consideration but, things like the local geology and water table are also an issue,” expands John Everitt. “In Royal Parade, a project we worked on in Cheltenham, we were only looking to lower the existing basement floor by approximately 300mm, but as soon as the contractor started digging, we hit a spring under the building so the tanking became far more complicated.
“If you need to create a basement or increase its depth you may also have to underpin the existing foundations of the property to ensure they go below the depth of the basement,” he adds. And in a highly built up area, you might have to underpin your neighbour’s home too.
“Creating a basement in ground that has a high water table can be particularly difficult and expensive,” continues Andrew. “The planning process normally requires structural design and technical surveys including ground conditions to address.”
But it’s not just below ground, it can be above ground too where you need to consider potential obstacles.
“Adding basements to listed buildings, or other historic settings such as conservation areas, can be very difficult and may not get beyond the planning stage,” warns Andrew. “The reason is that the new basement changes the nature of the original building, and the planners and Historic England are resistant to anything that changes the character of the original. It’s always best to consult with the planners before submitting a planning application in these instances.”
You’ll also need to think about how your garden is currently landscaped and planted. Are there any trees with Tree Preservation Orders that would potentially be damaged by an extended basement or additional foundations? Will you need to put tree protection measures in place? All these elements will add to time and cost when converting a basement.
And finally, there’s the issue of site access during the build. “You need to think carefully how the spoil is going to be removed from the site and how materials are going to be transported onto the site,” says Richard Gill, director at Paul Archer Design.
In a terraced house for example, this can often mean removing the ground floor windows in order to allow earth to be exported via a conveyor belt.
4. Could you face planning constraints?
Converting an existing cellar beneath a dwelling from a storage area to habitable space does not usually require planning permission. This will be covered by your home’s permitted development (unless, as previously mentioned, you are in a conservation area, special designated area, or your home is listed).
Exceptions can also include applications in areas designated as being Flood Zone 2 or 3, and in areas where there is a restriction in overall volume within local planing policy.
Reducing the floor level of a cellar to improve the ceiling height is, however, treated as an extension and so may need planning permission. And creating a new basement means you will certainly need it.
“You will need to apply for full planning in most areas and submit a Basement Impact Assessment and structural design to demonstrate that the new basement will not have an adverse impact on the hydrology of the ground and cause flooding or damage to neighbouring buildings,” confirms Andrew Mulroy.
“A Construction Method Statement may also be required, and while the architect and structural engineer can generally prepare this for simple projects, it may need the earlier involvement of a contractor for more complex projects.”
Does the Party Wall Act apply to basement conversions?
If the proposed works affect a Party Wall – eg. if beams are to bear onto a Party Wall; the wall is to be extended, altered, underpinned; or if excavations are to be carried out near to a Party Wall – the owners and leaseholders of both the building within which the proposed basement works are to take place and those of adjoining properties must be informed.
Professionals such as specialist basement contractors offering a design and build service will handle planning, building regulations approval and any Party Wall agreements as part of their service. Likewise, finding the right architect with experience in basement conversions will provide you with an invaluable source of knowledge and experience with these matters.
5. What building regulations will apply to my basement conversion?
The creation of a new habitable basement will require building regulations approval regardless of whether it involves a change of use of an existing cellar, or the creation of a new or larger basement through excavation.
The regulations are statutory minimum construction standards that ensure buildings are safe, hygienic and energy efficient. The renovation of an existing habitable basement, or the repair of a cellar that does not involve a change of use, ie. from storage to storage, is excluded from the building regulations. Full details can be found in the Approved Document - Basements for Dwellings.
In general, it is best to make a Full Plans Application for a cellar conversion, rather than to follow the Building Notice procedure, as this allows all design details to be resolved in advance of the work.
“Building regulations apply in full, and the design must ensure that the structural integrity of the building above is not compromised, and heat is kept in and water kept out,” explains Andew Mulroy, noting how basement ventilation can be tricky when it comes to building regs, especially as homes become less draughty with fewer fireplaces and old windows.
6. What budget do you have available?
As with any home renovation, extension or conversion, what you get as the end result is also highly governed by what budget you have available to you. Unlike building an extension, where some of the work can be done by someone with good DIY skills, basement conversions are far from being a DIY project. Throw in unexpected issues and you could find your budget spiraling out of control quicker than you thought possible.
Understanding your potential basement costs is key before you even start thinking about the finishes and design.
For converting an existing basement with adequate headroom, your costs will naturally be less, but once you start adding in digging out the ground, underpinning or even extending the basement you could be looking at around £2,250 - £4,250 per m2.
Other factors that can increase your spend could be the need to divert drains, your home being built on difficult ground conditions such as clay, poor site access, and as with every project, your geographical location in the UK.
FAQs
Can you extend an existing basement?
“Yes you can and we have done this regularly,” says Andrew Mulroy. “It is important to carry out good surveys and technical investigations so that you know what you are dealing with.
"Then you design the new basement extension using a similar construction as the existing, so everything works together structurally," he says. "Adding a new waterproof lining to the whole basement upgrades the property to current standards.”
Does a basement conversion always add value to your home?
“No, a poor basement extension can compromise the ground floor layout, the most valuable floor area in your home,” warns Andrew Mulroy. “But, a good basement extension can add useful floor area and drama to a home. In our Russet House project for example, the new basement was designed to create a dramatic atrium as well as provide rooms that could be used as bedrooms and living spaces to support multi-generational living that is a growing demand, especially in large cities.”
Is it always more expensive to convert a basement than build an extension?
“If the existing basement has the adequate headroom/ceiling height to allow for a suitable tanking system to be installed, as well as all the other main issues mentioned above, without the need for further excavation, it maybe a cost-effective way of creating additional space within a property,” says John Everitt, “but there are many factors that will affect the type of conversion and the costs.”
In general, for a large basement conversion that requires extensive structural work you could expect to pay twice the amount you would need for average extension costs.
Is it worth adding a basement?
Whether a basement conversion is financially worth it will depend on the cost of the work relative to local property values. It almost always makes financial sense to add lower-ground floor space in high-value areas, such as central London and other sought-after urban centres.
Elsewhere the potential to add value needs more careful analysis. Estate agents should be able to give you an indication of how much space is worth per square metre in your area.
Bear in mind that good-quality, well-lit basement living space with access directly onto the back garden will be worth considerably more than dark, converted cellar spaces with compromised headroom. It is also important to take into account the ceiling value for the location: remember, most suburban areas, and especially estates, have a maximum value that is very difficult to exceed no matter what improvements you make.
If you are weighing up adding a loft conversion against converting a basement, it pays to know that a straightforward cellar conversion costs about the same, but creating a basement from scratch is almost twice the amount per m². However, you may be able to create more useable floorspace with a basement than a loft conversion.
How long does a basement conversion take?
A simple basement conversion can be completed in a matter of weeks. Converting a single-room cellar using a membrane lining system can take just two or three weeks, including digging out the sump and fitting the pumping system.
Converting and extending the cellar beneath an entire house, involving underpinning of the existing structure, is however likely to take several months.
If there is access directly onto the garden or highway for the removal of soil and your existing ground floor is suspended timber, you can probably continue living in your home whilst work is under way. If the ground floor has to be removed and rebuilt because it is concrete, and all of the spoil has to be carried through the house, you will almost certainly have to move out.
While there are some big topics to explore before you commit to a basement conversion, such as damp in basements, John Everitt summarises the key points you need to work through.
“Look at the depth you will need to dig down, understand the local geology and water table, carry out a thorough services search to identify any services in the area and establish the depths of the drainage connections.
“Then, consider what you are going to use the basement space for – a gym, cinema, swimming pool, for example maybe far more suited to being housed in a basement than additional bedroom accommodation would be.
“Basement conversion ideas can be a great way of creating a double-height feature within a property, which can also help to improve their connection with (and to) the rest of the home,” says John, so it’s really a case of weighing up whether the rewards will outweigh the potential journey ahead of you.
