Basement extensions
Emily Smith discusses the considerations you need to make when when digging out a new subterranean storey
If you’re outgrowing your home, the best solution for adding space could be to extend down – Emily Smith investigates what’s involved
There are many reasons why people choose to dig down rather than extend horizontally. If you have a large garden in a rural location, then maybe a subterranean level isn’t right for you. But if you live on a tight plot in a built-up area that can’t accommodate a conventional extension, or already suffers from overlooking, excavating to create more space could be the best solution. Here are the main considerations for building down.
Potential complications
The thought of excavating a large hole under your property might sound like a recipe for trouble, but the reality is that adding a basement could actually increase the structural stability of your home. “Many historic properties were built on poor ground, with badly designed foundations that aren’t in line with today’s standards,” says Neil Dusheiko, Director of Neil Dusheiko Architects. “Some older dwellings might have suffered from damp and settlement because of their grounding.” Geological surveys and soil tests can be used to establish the conditions beneath a house, which should be fed into your structural engineer’s drawings. They should then produce a construction method statement that details the design of the new basement, foundations and waterproofing, as well as how work will progress on site.
“If the condition of the building is sensitive, the contractor should set up monitoring stations to check if any movement is taking place due to the works,” adds Neil.
“Complications can arise if you’re working with an inexperienced construction team or under-equipped design. It’s only in these rare scenarios that your project might be
at risk of suffering avoidable issues,” says Jake Puddy from London Basement. Retrofitting a basement is inevitably a complex engineering project, but you should feel safe in the knowledge that you’re working with experienced professionals who have conducted the appropriate research to form a design that is in line with modern building standards.
One of the most common hurdles is dealing with unhappy neighbours. Your project could cause noise and disruption for several months (if not longer), which in turn could lead to tension. Plus, if your house is a terrace or semi-detached, the work could directly affect those next door – especially if you need to temporarily support boundary walls or the adjacent road. The Party Wall Act safeguards your neighbours by requiring you to notify them at least two months before excavating. If they aren’t happy with the proposed works, then you may need to get a surveyor involved.
Planning the design
Straightforward cellar conversions or work on pre-existing basements usually comes under permitted development (PD) – meaning you don’t usually need planning permission. Retrofitting a subterranean level that will add new accommodation or change the appearance of the house, however, is likely to require formal consent. Some local authorities in towns and cities are removing PD rights for basement extensions due to the problems with so-called iceberg basements (huge multi-million pound, luxury additions that are set over several underground storeys).
The design of the space is crucial to its success. “All too often basement rooms are lifeless areas starved of natural light or a connection to the outside because the design hasn’t been properly considered,” says Chris Romer-lee from Studio Octopi. Some people might assume basements to be dark and dingy zones, but in reality, the right balance of illumination and good ceiling height (ideally at least 2.6m) will create bright and spacious rooms that feel connected with the rest of the house.
Your design and layout should be based around how you’re going to use the space. For instance, are you creating extra room for general living areas or will the basement play a specific role, such as a cinema, gym or spa zone? “Typically, people want their basement to be a new bedroom, family room, bathroom and utility,” says Jake.
Don’t think about the basement as a separate space, but instead as another room of your home, just like a horizontal extension would feel. Aim to create a seamless flow between the storeys by matching flooring and style. The positioning of the staircase will also have an important role to play. Tucked away flights will make the lower ground feel like a secretive area, whilst a more visible design will aid the sense of connection and could become a dramatic feature. “We tend to follow the existing
staircase, for good symmetry,” says Jake.
Getting the light right
Approach artificial lighting in the same way as you would anywhere else in the home by concentrating on how much illumination is needed for the rooms to function as they should. When it comes to filtering daylight into the basement, light wells (a channel that leads up to a glazed panel higher in the building) are a good solution, as are sunpipes and walk-on rooflights on the ground floor. Glass balustrades on your staircase and glazed walls between areas are simple solutions that will assist in the flow of daylight. Remember to zone your lighting arrangement around how you’re going to use the space – for instance, you probably don’t want a glazed panel shining daylight down onto a large cinema screen.
The works needed are probably going to make a mess of your garden, so consider extending out under the outside space to create an even larger addition – you can simply re-landscape over the top of the storey. For maximum light gain, dig out a sunken courtyard that leads up to the ground floor garden. This will allow you to integrate glazed doors and will offer a connection with nature – the right design will offer the illusion that you aren’t underground.
Project logistics
Digging down is going to displace a lot of soil, which will all need to be transported out of the excavation – a conveyor belt will probably be used to shift the unwanted earth out of the way. You’ll also need temporary props to hold the house in place while your team digs and forms the walls.
The services for your house are likely to be positioned right where you are looking to excavate. This could include your property’s drainage, electrics and water supply. “This is actually very straightforward to deal with, as they will simply be integrated into the ceiling of the new basement, meaning they don’t need to move too far – we normally look to upgrade them at the same time,” says Jake.
Once you’ve removed all the soil and built in any structural supports and foundations, you’ll need to create a watertight space. There is always going to be moisture present beneath the ground, so it’s usually a case of managing the flow rather than blocking it out completely. “On retrofits, this typically involves the installation of an impermeable membrane to the internal face of the new concrete substructure,” says Jake. “We use a type C cavity drainage membrane – there are various products available, but there’s no real difference in price.” This is essentially a large, sturdy bubblewrap covering applied to the floor and walls that allows moisture to trickle through studs and gather in a gulley at the edges of the basement, before being channelled into the mains drainage system via a collection sump and pump.
Other options include water-resistant concrete wall construction (mainly used in new build basements) or the application of a waterproof membrane to the outside of the new foundations, which can be good in plots with high water tables. In many cases, a combination of waterproofing methods is specified to deal with conditions on site and to gain a warranty. With the waterproofing completed, the rest of the project is much like any horizontally built extension. As long as your design has been well-planned to provide a good balance of natural illumination and artificial lighting, with a strong connection to the rest of the house, then your new storey will add both value and space to your home.