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Home & garden.

Updated: 29 Jun 2021

Basement conversions and extensions

Considering a basement conversion? Here's what you need to know.
Rachel Christie
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The premium on space, especially in cities, has seen interest in subterranean spaces soar as they become elevated from basic storage rooms to beautiful, fully functional living spaces.

A basement extension can be a clever way to expand your living space without taking away any garden space. Whether you fancy adding an extra bedroom, kitchen diner, or cosy snug, we take you through everything you need to think about, from planning permission to the pros and cons of creating a basement.

What are the pros and cons of building a basement?


  • When you gain space from a basement you don’t lose any external space. Plus, it will free up space in other parts of the house.
  • Securing planning consent is generally easy, especially if there are already basement conversions in your street, and if you have windows or light wells outside.
  • In urban areas, a basement can add value.
  • Underpinning can actually stabilise an old building.
  • Waterproofing (or tanking) makes the whole house drier and healthier and increased insulation makes it more energy efficient.
  • It can be completely self-contained with its own access.


  • The job is expensive and complex.
  • It’s bound to be disruptive, noisy and messy especially if you are enlarging or creating more headroom and so there's the need to underpin.
  • You need to have an annual (at least) inspection of the pump and drainage.
  • There may be unforeseen problems, especially if there is a party wall.
  • You may not be able to get enough light in and the room may end up gloomy or always dependent on artificial light.
  • Safety may be a concern and you may end up having to lay out more money on another access route.

Here are some key questions to consider before going ahead:

Should I convert my basement?

If you're lucky enough to have a pre-existing cellar, it may make economic sense to exploit it by turning it into a habitable basement. If you don't, the process is trickier and potentially not possible.

The costs and complexity of the project will vary dramatically depending on whether you have an existing cellar or basement with enough headroom that can be easily converted; a basement conversion is likely to be much cheaper if it's an existing space with no structural changes needed. Or whether you will need to fully excavate beneath the house to make a basement possible.

The most cost-effective option is a simple utility basement with no windows, which would work well for a workshop, utility room, wine cellar, or just for extra storage, freeing up above-ground space for other uses. 

With some investment in electrical and plumbing works, and finishes to the walls and floor, the basement space can be upgraded to make a games room, home cinema or gym; all possible without the need for windows.

If you're after a full lower ground floor with plenty of lightwells, your project will most likely require excavation and underpinning, which will be expensive. However, a functioning subterranean level could provide more functional rooms like a kitchen, office, guest bedroom and bathroom.

With a separate access via an external staircase, a basement could even be used as the home-based business or a self-contained annex.

Do basement conversions add value?

If you have outgrown your living space but you're keen to stay put, building an extension is a great way to make more of what you’ve already got. Adding an extra room, such as a converted basement, can increase your property value. 

We recommend you speak to a couple of local estate agents before you start; they will be able to indicate whether you will recoup your investment when it comes to selling. 

A good source of natural light and decent ceiling height will help to ensure that your conversion is a worthwhile investment that boosts your home’s value.

Do basement conversions need planning permission?

If you are converting an existing cellar and are not making any changes to the external appearance of the building, you are unlikely to need planning permission. But you will almost certainly need planning permission if you are making any structural changes or altering the external appearance of the property, if the building is listed or in a conservation area and if you are creating a separate unit of accommodation.

If your basement extension requires excavation to create more room or if  you are making changes to the external appearance of the property by adding a lightwell for example, then you are likely to need planning permission.

Planning rules are continually being reviewed and they vary considerably from one area to the next, so it is a good idea to contact your local planning authority at an early stage and well before any work begins.

Do basement extensions need building regulations approval?

Regardless of whether you need planning permission or not, you will almost certainly still require building regulations approval to ensure that  your conversion meets a minimum standard of health, safety and welfare conditions; this covers areas such as ventilation, fire safety, foundations and energy efficiency.

It may seem like there are an overwhelming number of professionals to consult when it comes to planning a basement extension, but speaking to a reputable local builder with experience of similar projects is the best place to start. Head to Which? Trusted Traders to find a reputable builder near you.

Your builder, architect or planning consultant should be able to provide you with a package of plans and structural drawings. They'll also work alongside local authority building control in order to achieve building regulations approval. 

The structural drawings will be provided by a structural engineer who will also produce technical drawings. 

We've spoken to a builder and an architect for their best advice when it comes to tackling any sort of extension. Log in now to unlock their insight about what you should and shouldn't do.

If you're not yet a member, join Which? to get instant access.

Do I need a party wall agreement?

If your basement conversion requires structural changes to the building, such as excavation, you will likely require a party wall agreement. Some of the main criteria for requiring a party wall agreement includes:

  • Excavating within 3 metres of your neighbour’s property and to a lower level than their foundations.
  • Excavating within 6 metres and at an angle intersecting 45 degrees from the bottom of their foundations.
  • Cutting holes into a shared wall to insert beams or flashing

Party wall agreements aim to protect neighbouring properties during and post construction and in most cases will involve the serving of relevant notices and legal agreements to neighbours. We have more information on party wall agreements on our building regulations and planning page.

Extending a property can often be a contentious issue with neighbours, so you should ensure that you contact a party wall surveyor at an early stage and warn your neighbours as soon as you can.

How high does the basement's ceiling need to be?

Building Regulations do not specify a minimum height for ceilings, but 2.4 metres is a practical height to aim for. 

You will need at least 2 metres over the stairs to keep within building rules. Don’t forget that you will need to include space for the height of your flooring within your calculations.

How long does it take to convert a basement?

Depending on the size of the property, the amount of excavation and building work required and its complexity, the work will typically take at least 12 weeks and could go on for much longer.

What's the best way to maximise light in a basement?

  1. Light should be a major factor in deciding the basement’s layout. Position habitable rooms so that they get the best of the daylight and any outdoor access.
  2. Ideally the landing area will have some natural light, most likely from the hallway above, to ensure safe passage.
  3. Space near the centre of the floorplan or on elevations with no lightwells can be utilised for cloakrooms, bathrooms, storage spaces and utility rooms; all of which can function without natural light.
  4. Consider a sunken courtyard garden, with glass doors accessing it or glass balustrades and open tread staircases to help light flow down from the floor above. Plus, glass partition walls and glazed or partially glazed doors allow light to filter through between spaces.
  5. Where a basement extends beneath the garden, flat rooflights are a great way to bring in extra light.
  6. Use neutral colours and finishes to enhance light and space.