The Absolute Basement Design Guide

Using your basement to increase the space in your home without affecting the exterior of your home, is a truly great solution. The new space can be used in many different ways such as a kitchen, guest room, living room or perhaps a gym.

basement conversion



According to CABE, the Commission for Architecture and the Built Environment, the UK builds the smallest new homes in Europe. This is mostly due to the fact that the cost of building a new room is very high, let alone a large home. However, a basement conversion will be beneficial in allowing you to have more space for living and storage without building up or out, or impacting the landscape.

Benefits of Adding a Basement

It’s a simple and wise solution, and far from a new idea: basements and cellars were standard on most townhouses in the 19th century and this space is now highly prized and rapidly being renovated, extended, and converted. Basements are particularly popular in higher value areas because the need for additional space is higher. Many homeowners are even excavating the area beneath existing homes, making room for a basement.

There is a tendency for home-owners, specifically those the engage in DIY, TO include basements in the new construction of homes. The inclusion of basements in new homes is largely driven by the need to get more built volume out of an expensive site, restricted either by physical size, or planning policies on footprint area or ridge height.

This, in turn, is leading to great improvements in design and build quality, and is helping to bring down costs. A basement also increases the value of your property and makes it more appealing to the eyes of potential buyers. Designed well, a basement can provide considerable extra space at a lower cost when compared to other house extensions.

Common Basement Designs

The most cost-effective option is a simple utility basement with no windows, ideal for a plant room, workshop, utility room, wine cellar, or for storage, freeing up above-ground space for other uses. With a little investment in electrical and plumbing works, and finishes to the walls and floor, the basement space can be upgraded and used for many purposes. Some people try to turn the entire basement or a potion of it into a gaming room, home cinema, or gym. While some people may be concerned with the a lack of light making it into the basement, large windows and lightwells make this a non-issue.

If you want to really make use of your basement to expand your living space, then it will be necessary to start incorporating lightwells and fire escape stairs or ladders accessible from each room that will be used.

Basements that will provide plenty of light via lightwells and large windows may be used for a family room, kitchen, office, bedrooms, or bathrooms. Keep in mind that even if you opt to turn your basement into a more livable space, you can still include the usual utility functions. With a separate access via an external staircase, a basement could also be used as the office of a home-based business.

Light is Key

Light is an integral part to creating a basement that will not only function properly, but is a space you want to spend time in. Creating a space that is both quality, enjoyable, and functional is key when looking at a basement design guide and deciding on a basement’s layout. Habitable rooms should get priority of daylight and any outdoor access, and should therefore be positioned accordingly. Ideally the landing area will have some natural light, most likely from the hallway above, to ensure safe passage even in the event of a power failure.

Space near the centre of the floorplan or on elevations with no lightwells can be utilised for circulation or for cloakrooms, bathrooms, storage space and utility rooms, which can function without natural light.

How to Maximize Incoming Light:

  • Install multiple lightwells, especially on southern elevations
  • It may be good idea to implement a sunken courtyard garden, with doors accessing it
  • Designing a large open plan stairwell through the entire building that is lit from above by a large rooflight will help bring in a lot of natural sunlight
  • An open plan layout will allow daylight to reach many points of your home, including the basement
  • Glass partition walls, glazed or partially glazed doors, and glazed toplights above doors allow light to permeate between spaces
  • Consult an expert to make sure you gave a good lighting scheme, as this is essential to your basement overall
  • Use neutral colours and natural finishes to enhance light and space

Basement Design Guide: The Shape

The best way to ensure you aren’t accruing unnecessary costs is by keeping the basement design the same shape as the ground floor footprint. This will make it so no further foundations will be required to support the house. Where deep trench-fill foundations are being specified because of ground conditions, going a little deeper and excavating a basement is worth exploring.

Straight walls are the most cost effective option, as curved walls will require extra time and money. However, if you are set on having curves in your wall it is best to use insulated concrete formwork.

Larger basements achieve greater economies of scale, as the cost of the structure is spread over a greater area. So, if you decide to build a basement you may as well extend it all the way beneath the house, and possibly beyond into the garden, too.

Ceiling Height

basement-conversion-ventilation-heatingImplementing tall ceilings will improve the quality of space, especially when it comes to larger rooms. Designing the basement to have taller than
average ceilings will help balance out the proportions of the room. However, keep in mind that the deeper you dig, the greater the cost, which is why it is important to keep balance in mind.

There is no minimum ceiling height for basement ceilings under the Building Regulations, but a practical minimum height is 2,400mm, and the taller the ceilings the better. If the basement is to form a separate dwelling, then most local authorities will apply a minimum ceiling height when determining the planning application of 2,300-2,400mm. Remember to plan for insulation and screed build up over the floor slab, plus an allowance for whatever flooring finish you are choosing to apply on top.

Do not forget to plan for drainage runs with sufficient falls to reach the sump, which will also influence the position of the wet facilities and the floor height.

Lastly, the ceiling will need to accommodate services for lighting, ventilation, and plumbing, and so a service void will be required, the depth of which will be dictated by duct sizes, choice of light fittings, and whether you opt for a suspended precast concrete floor or suspended timber floor.

 Bathrooms and Utility Rooms

It is imperative for you to understand the sewage system (or bring in an expert) when deciding to use your basement for a bathroom or utility room. Removing waste from bathrooms, kitchen sinks and washing machines relies on gravity, but in a basement it is more than likely that the sewer level will be above the slab level and so all foul water will have to be collected in a sump (which will need to be sealed and vented) and pumped up into the sewers (or off-mains drainage tank).

However, this is not usually an issue for basins or WCs, where drains can be boxed in against a wall or within a service duct. It can be a problem from baths where the waste is lower, as well as low-lying shower trays and walk-in showers. A common solution is to build up the floor level within bathrooms and to limit the distance to the sump tank.


It is important to note that if you are renovating your home and the ground is being excavated, the walls that are currently in place will need underpinning and more support for the flooring, using timber or steel beams. This will require design and calculations by a structural engineer. The structure will also have to meet a minimum half-hour fire rating.

Fire Escapes

In the event you are opting to use the basement for bedrooms or other living spaces, you will need an external door or window suitable for an emergency escape. This should measure no less than 0.33m2, measuring at least 450mm x 450mm. Alternatively, a protected stairway leading from the basement to a final exit can also serve as a means of escape in the event of a fire or flooding.

Air Flow

The Building Regulations require ventilation to be designed to take into account moisture content in the structure and air quality in the basement. Mechanical ventilation is the most likely solution unless there are sufficient external doors and windows. It may be a good idea to consult an expert to ensure you are adhering to the guidelines properly.


basement stairsStairs will be a necessary addition to the basement and should have a pitch of not more than 42° and have a headroom of not less than two metres,
with suitable handrails and non-climbable balustrading with a gap of less than 100mm between spindles. Location of stairs needs careful consideration for space efficiency.


In order to ensure the basement space is habitable, insulation should be provided in external walls and floors. The insulation should have a reasonable thickness and be compatible with the tanking material. All doors and windows must comply with Part L with a U-value of 2.0W/m2K or less.


Waterproofing your new basement is another important part of the design process that should be approached with careful consideration. Whichever method you go with should be appropriate for resisting ground water pressure and should have British Board of Agreement (BBA) or similar independent technical accreditation. Most basement specialists use a cavity membrane system. This is a textured breathable membrane around the entire basement behind which any damp is trapped and channeled down the outside of the basement walls, under the floor and into a sump from where it is pumped harmlessly away.

Planning Tips

Planning permission for a basement within a new home should be straightforward in most instances. Exceptions 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 plan policy. Be sure to check with your local authorities to ensure you are properly adhering to all planning rules and regulations.

A new basement design beneath existing dwellings are likely to be considered as Class A Permitted Development and, therefore, will not require planning permission in most instances subject to design constraints on lightwells and size. However, there is no hard and fast national planning guidance on this currently, and no consistency of approach across the various planning departments.

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