basement extension design guide


Good Basement Design:

Basements are becoming more and more popular as a way to add space and value to a home. As property prices have increased so have the number of projects where a basement makes good financial and design sense. However, perhaps more than any other type of project, it is important to get the design of a basement right and to understand the best way to do things.

Basement extension design guide


extending down for more space


Things to keep in mind

Creating a basement can be a fantastic way to add space to your home, whether that is for an extra living space, gym, cinema room, bedroom, or even swimming pool. A basement can also often be the only way to achieve permission to extend a house on a tight or constrained site.

Basement contstruction is roughly twice as expensive as building above ground though, so it is important to plan the work properly. In this post we will cover many of the key things to keep in mind to make sure your basement project goes smoothly, from planning to construction.

extending underground

basement design guidance


Basement Popularity

Over the last 10 years the number of projects we put on site involving a basement has risen very significantly. In fact more than 50% of the work we put on site now involves a basement of some kind. There are a few clear reasons for the increasing popularity of basements including:

  • House prices have gone up so much that the cost of building a basement is now quite sensible, and will add value to the house.
  • Basement construction has improved, modern water proof concrete and tanking membranes keep basements dry and warm.
  • The extra space that a basement adds is seen to have a big value, as people extend their homes rather than take on costs of moving.
  • Modern life calls for more utility space and store rooms, as well as more living space for big families, all ideally suited to basements.
  • Planning for extensions has got more difficult, but basements can be easier to get through as they are less visible, and can be PD.

Basement Projects

what rooms can you create in a basement?


Typical Basement Design Projects

With good design, a basement can transform a house and add lots of space and value. Of course, basement design ideas should be unique to each project, but here are a few we have worked on:

  • Guest bedroom and ensuite
  • TV / cinema room and bathroom
  • Garden room with stepped terrace
  • Living room and kitchen
  • Swimming pool
  • Gym and changing rooms
  • Utility, store and laundry room
  • Library and study


Guide To Basement Design

basic requirements for all basements


Basement Design Requirements

Whatever basement design ideas you pursue, keep in mind that there will always be a few basic requirements. Make sure that you use a good architect with experience in basement design, that will be able to make sure that all of these points are considered from the earliest design proposals. It can be very costly to make mistakes with basement design, so these are all important points to keep in mind from the outset:

  • Waterproofing and Drainage.
    A successful waterproofing and drainage design is fundamental to all basement projects, the system should have a warranty of at least 25 years.
  • Structural Stability.
    A good structural engineering design is vital to ensure that there is no movement to the existing house and the finished basement design is structurally sound.
  • Natural Light.
    It is important to design a basement with natural light to all of the rooms that you want to be considered as habitable spaces, such as bedrooms and living rooms.
  • Ventilation.
    It is always preferable to design a basement with natural ventilation and fresh air. Otherwise you’ll need an air conditioning system and plant room but that adds cost.
  • Access.
    Proper access to the basement is important. Keep in mind that a new staircase down might also mean that some changes are needed some to your ground floor plan.
  • Means of Escape.
    It is very important to be able to get out of the basement safely if there is a fire. This could be through a light well, or a protected route up to the front door.


Basement Planning Applications

approval process for basement design


Planning Approval For Basements

Planning authorities have tightened their restrictions on basements and require additional information with any basement application. However many people are surprised to learn that planning permission for basements can still be easier than for some other types of project. This is because a basement will have no impact on the appearance of the house above ground, and a basement does not create overlooking or affect a neighbour’s amenity directly. In fact, if the basement is just under the house (not the front or rear garden) it can even be Permitted Development. To satisfy the planning department a well written Basement Impact Assessment will need to be submitted as part of the planning application.


Basement Impact Assessments

A Basement Impact Assessment or BIA is a document that Councils (esp. in London) ask for as part of a planning app. A Basement Impact Assessment will include details of the site and ground conditions, any trees, the water table and structural design of the basement. It will also set out a plan for how the basement will be constructed, impact on neighbours, the street and traffic. A good architect can guide you through how to get a BIA right. You will also need a structural engineer and a specialist BIA firm for the report.


Party Wall Awards for Basements

Most basements will need at least one party wall award with a neighbour, and often more than one. They key to get a party wall agreement for the basement is to have a good, clear structural design. You then need to serve the right party wall notice to get it all agreed. We have a post on Party Wall Awards for House Extensions that takes you through it step by step.


Basement Detailed Design

glossary of technical terms


Basement Technical Design Details

Basement construction is pretty technical, so it is likely that you will come across a few terms you may not have heard before. Here are a few terms that will come up on pretty much all basement projects.


Reinforced Concrete

RC concrete should just be called RC as this stands for ‘Reinforced Concrete’. Most basements will use some RC. The concrete is reinforced with steel bars that are laid on site and the concrete poured around them. It makes the concrete much stronger and is almost always used in basement floors and walls.


Water Proof Concrete

Although concrete is strong it is not actually completely water proof. Over time and under pressure it is possible for water to seep through solid concrete. Water proof concrete is different as it uses a chemical admixture to change the chemical structure of the concrete and make it water proof. Best used with RC.


Basement Tanking

Tanking is a short hand term for lining the inside of the basement with a water proof membrane. This is usually a semi-rigid plastic sheet with egg-crate-like dimples. The idea is that any water that makes it in will be caught by the tanking and will then be taken away by the drainage system. This type of thing is also referred to as a cavity drain system.


Basement Sumps and Pumps

It is common for basements to be lower than the drainage pipes and sewers serving the house. So, any drainage from the basement needs to be pumped up and away through the drains. Sumps are heavy duty plastic chambers set below the basement that contain pumps and connections to the drains. You may need 2 sumps, 1 for surface water and 1 for waste from bath rooms. Each sump can contain 2 pumps (in case one fails), alarms and battery backups.


Basement Extension Project Guide



Basement Design Tips

Great basement design is not rocket science. But it does need a good understanding of what you can and can’t do. These are 7 of the best design tips to follow to get your basement project on the right track.


1. Double Up Your Basement Waterproofing

Use 2 separate systems for waterproofing, the cost is far less than having to repair the work in the future. There are 4 main systems to choose from.

  • Water Proof Concrete (a special chemical additive in the concrete mix).
  • External Membrane (hard to install unless the basement is new build).
  • Internal Membrane (catches any water that comes through the concrete).
  • Water Proof Render (easy to put on the inside, but can fail easily too).


2. Plan To Create Lots of Natural Light in Your Basement

A good design should maximise the natural light and ventilation to the basement. The size and positions of light wells should be carefully considered; and if possible landscaping can be stepped down so that the basement feels like a bright, fresh space. Try to achieve some views of the sky from the basement, and work out where the sun will shine in to the space to make the most of it in the design.


3. Get The Maximum Head Height You Can For A Feeling Of Space

You can almost always control the head height of a basement. Dig deeper, get a better head height, but it costs a bit more. Go for at least 2.5m as a finished head height. It will make the space feel much bigger and brighter and is almost always worth the additional cost. This is not an area to scrimp. You can’t change it once it’s done so go for it while you can.


4. Assemble A Really Good Basement Design Team

A basement is not a good self-build project. It is complex and specialist, and you need the right people working for you. We can not stress this enough. Don’t risk the expense and stress of a basement project gone wrong. There is no substitute for a good professional architect who will have completed the design and will be able to administer your contract for the work on site.


5. Use a Solid Building Contract For The Work

This goes without saying, but the contract you use with your builder should be fixed cost and time. Your architect will almost certainly advise the use of a JCT or RIBA contract, which are both great and do the job. There should only be certain conditions where any extra cost or time are justified, and only if your architect instructs on this.


6. Get A Highly Experienced Structural Engineer

A basement extension is, at heart, a structural project. It is likely to involve underpinning your house and lots of heavy RC concrete work. Not to mention the temporary structural work to keep everything safe while the basement is built. This is all firmly in the remit of a structural engineer, and you will want the best you can get. Your architect is likely to be able to suggest a structural engineer that they know and trust.


7. Always Use a Specialist Basement Contractor

This may be one of the most important points on the list. A basement is not an easy build. You do not want a builder that does not know what they are doing. We will repeat that: you do not want a builder that does not know what they are doing. There are loads of builders that just do basements day in day out and have the knowledge that you need. There are also lots of big building firms that do basements as well as other work. Both are fine, but don’t use someone who has not worked with basements or RC concrete before.

More Guidance

all posts

We have written a number of posts which provide information and guidanceto anyone considering a project. These include advice on design and planning of new homes, renovations and extensions.



If you have a project you would like to discuss with us then just drop us a line using this form and we will get back to you. Or call us in our London studio on 020 3920 9805.

We will be happy to help and we look forward to hearing from you.



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