Basement Conversions London | Ideas, Designs & Costs Basement Conversions London | Ideas, Designs & Costs

Basement Conversions And Extensions

This blog outlines some of the key things you should consider if you plan to convert your cellar in London.


The complexities of a basement extension mean it can be labour intensive, so we recommend hiring the right people to keep costs under control and help you plan; see our fees here.


Getting Started with your Basement Conversion


Basement conversions and extensions in London are increasing in popularity, mainly because the area is short of buildable space. Expanding the floor footprint of the property is critical for creating more space in a growing family home. From a developer’s perspective, increasing the size of the property with a basement conversion is a great way to add substantial value and make a considerable return on investment.


We usually see these types of rooms used in basement conversions:

  • Cinema Room
  • Gym and Exercise Rooms
  • Reception Rooms
  • Swimming Pool
  • Sauna
  • Utility Rooms
  • Wine Cellar
  • Home Offices

How much does a basement conversion cost?


As mentioned above, basement conversions are an excellent way to increase the value. You don’t have to be a property developer to want to see a return on your investment! But how much value will it add? It isn’t easy to calculate and depend on many different factors such as your area, quality of finish, and more space.


In very built-up sought-after London areas such as Kensington and Chelsea, Richmond or Hammersmith and Fulham, space is becoming harder to come by, meaning every square foot on the floorplan will add extra value. As most of these homes cannot build up and out, basement conversions are a great idea. Doing your research on other homes along your street and in your postcode will give you a good idea of home much value it could add. Calculating the conversion cost for a basement extension can be slightly more challenging to work out. Below is an estimation of how much the build cost for a basement conversion could be!

Basement (under existing house plus underpinning): £2,250-£4,000 per m2 of additional floor space


Site accessibility will play a large part in your basement extension hence the wide range in cost

Basement (under garden): £2,000 – £3,000 per m2, additional floor space


This can be a great option in conjunction with walk-on roof lights to let light into your new basement space


Wondering how much your basement conversion will cost?

We've written a blog which will let you calculate the costs.

Read here

What surveys you could need for your basement conversion:


To limit all risks, completing several different surveys will help build a great understanding of your limitations. This will help make a more robust Planning Application and give your engineers the correct information to design your basement safely.

The surveys we recommend completing are:
  • Geo-technical Survey
  • Flood Risk Assessment
  • Drainage Survey (to check the condition of your pipework – as per image)
  • Topographical Survey
  • Arboricultural Survey (if you have nearby trees)


Carrying out these surveys will provide your Architect, the planning officer, 3rd party consultants and the contractors the information they need to carry out due diligence of any restrictions, estimate all the costings and limit any mistakes.

How to add natural light to your basement conversion:


Adding light is one of the more difficult challenges you may face when converting a basement. Lightboxes will be your source of natural light, so planning these strategically is important so that the room is filled with light and is not dark and dingy. Designers are full of basement conversion ideas and will strategically place the interior spaces and lightwells to maximise all opportunities for natural light. Basement conversions in London are particularly challenging, but the results are very much worthwhile.

Strategically placing rooms that do not need natural light will mean you can optimise the rooms which may need it. For example, adding a utility room or wine cellar in a problematic area to light means that you can place a living room or bedroom in an area with access to the window.

Minimising the number of interior walls decreases the amount of light that is being blocked off. This depends, of course, on your brief and needs for the project. If an open plan does not work for you, glass doors and separating walls will create separate spaces and create an airy feel.

Basement conversion ideas

Key Point: You are likely to face further restrictions to what you can do if you are within a conservation area or have a Listed Building

Achieving planning permission for your basement conversion:


Every planning office will have it’s own policing regarding cellar conversion, so you must investigate and research your Borough thoroughly. Using an architect who has an excellent planning approval success rate and is local to your area means they should have previously worked alongside the planning office. Most London Boroughs now require a Basement Impact Assessment before considering your application.

This would include most of the surveys described above and is typically carried out by a structural engineer. Going through the process of design, drawings, and planning application can be a tiring and expensive process as it is.

Getting the approval declined can often mean you have to go back to the drawing board, which can come at more delays and an increase in costs. Hiring the necessary specialists will help cover all grounds and leave no stones unturned, building you a robust application – an experienced architecture firm like AURA Architecture will do this for you and recommend the correct guidance.

Thinking about converting your Basement?

We have worked with Boroughs across London, and specialise on working on Residential Properties

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Methods to waterproof basements:


Waterproofing is a critical and vital stage of any basement conversion. It is essential that it is done correctly and no corners are cut. A leaking basement is a complicated issue to solve, and the basement conversion cost can rocket as it is not always easy to locate the origin of the leak – stress that you do not need! We recommend that you find a Waterproof Design Expert from the start to help you with the design process.

Through the rest of the project, your Architect should help you find and liaise with an experienced Waterproofing specialist. Water Proofing is a very skilled trade so hiring and finding the correct experts and construction companies is extremely important. Be sure that you are provided with the necessary Paperwork and Certification. This will help you with guarantees (should you look to sell the property) and be presented to your insurance provider (if the dreaded happens, and you get a leak). Typically a manufacturers warranty of 30 years will be provided. We are experienced in using DELTA Waterproofing systems.


There are 3 main methods for waterproofing:


Type A – External Waterproofing: Great for new builds with lots of space around, it is a really effective way of preventing leaks. However, it needs a lot of space and cannot be down with neighbours close by. Therefore it is usually not appropriate for London conversions.


Type B – Waterproofing Concrete/Slurry: Concrete is positioned and laid around the perimeters to prevent the water from coming in. This is a suitable method for water prevention in London, although we usually see this leads to leaks after time.


Type C – Cavity draining tanking system: This is a draining system that gutters all the water in a trench around the perimeter of the conversion and pumps it out. We would recommend using this method as it is often more reliable as it deals with water issue rather than just preventing it.

Basement WaterproofingBasement Waterproofing

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    Basement Conversions in London Ben Richards
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