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Planning your Basement Conversion

Kensington, London Basement Hallway entrance

Converting your basement is the best way of adding living space to your home without drastically altering the exterior. Unlike a loft conversion, it is close to the communal rooms in the house which means it’s more flexible and can be used for family spaces, bedrooms, entertaining spaces, or even running a home business.

Conversion Types

There are a number of different basement conversion types so you have the flexibility to build the basement that best fits your requirements. Your type of conversion (and whether or not you need planning permission) depends on your existing basement space and what you’d like to do with it. The largest difference between the different types is the square footage that you can achieve.

Under House (Retrofit)

If you already have a basement that’s just used for storage, you can convert it into a living space without the need for planning permission. Head height can usually be increased to a suitable level without the need for major underpinning works. These conversions are the fastest and simplest since the space is already there.

However, if your home does not have a basement it is likely that you will need planning permission – especially if you plan to add light wells to the exterior. Most of these basements are under house or retro-fitted, meaning that they are excavated directly below your property.

Retro-fitted basements are the most popular as they can almost always be added onto a home without interfering with underground pipes or experiencing issues with planning permission. These can be built as full footprint conversions (which extend through the full area of the house) and half footprint (which are about half the area of the house).

The size of your basement conversion depends largely on your requirements, budget, and preferences. Some rooms may not require a lot of space, while others have minimum requirements – especially if you would like to build a subterranean garage or swimming pool.


Sub-basement conversions are excavated under houses that already have a basement, but would like to expand further. This creates a multi-storey basement and offers yet more room for expansion. In some instances, it can be difficult or impossible to arrange a sub-basement conversion, depending on what is underneath your home. If you live close to transportation links or there are essential water pipes under you home, you may only be able to have a single-level basement conversion.

Under Garden

Under garden conversions create a true iceberg home by excavating well beyond the walls of the home to create a basement conversion that extends throughout the garden. This means that the basement of your house will be considerably larger than any of the other storeys. It also offers more opportunities to get natural light into the basement since roof lights can be fitted to get light into various parts of the room, rather than relying on a single lightwell at the front.

Natural Light

Nobody wants their basement conversion to actually feel like a basement, which is why our design team puts so much emphasis on letting in natural light. There are a number of ways to let in light from outside as well as arranging electrical lighting options to make the rooms as versatile as possible.

Light Wells

A light well is a short trench that’s dug alongside one or more of the basement walls to let natural light filter down to windows installed in the basement. Many clients prefer to excavate large light wells that serve as a small patio area with plants etc. so that there is a pleasant view from the basement and some outside space.


In some instances, the top of the basement is actually above ground – this is particularly common if your home is built on a slope. Adding some windows to your conversion will vastly increase the amount of natural light that comes into the room.

Light Tubes

Light tubes work by redirecting light from ground level through a reflective pipe and into the room. These can be installed in most basement conversions, but are particularly common in under garden conversions where the ceiling of the basement is just under the garden and light tube installation is much easier.

Glass Doors

If your basement has its own entrance, you may want to consider glass doors or frosted glass to let in more light from the outside. There are a number of options that use toughened or reinforced glass so your home is still secure.

The amount of light you need in your basement conversion really depends on the usage. Bedrooms and living areas need natural light to feel comfortable, while rooms like home cinemas or other entertainment spaces often benefit from restricted light. Our design team are available to provide advice and draw up plans based on your usage and the type of basement conversion you require.

Basement Kitchen, Clapham,London

Wine Cellar, Basement, West London

Planning Permission

In most instances, your basement conversion will require planning permission. We always ensure that your basement conversion gets as much natural light as possible, so there may be minor changes to the exterior of your home to create light wells for the basement.

If you prefer to work with your own architect or to submit planning applications yourself, we will provide as much support as you require to ensure that the process goes smoothly. Only around 10% of planning applications in London are rejected, and resubmitting the application with minor changes often ensures that the project can continue. However, this process can be time consuming which is why we often recommend that you work with our partners to benefit from their proven track record.

Detailed Drawings

Our architects will put together detailed drawings for your new basement and carry out any necessary investigations such as tree reports, noise surveys, and flood risk assessments. These must all be submitted together to provide all of the information necessary for your local authority to make a decision. The information is also essential to ensure that the build can continue without unnecessary delays or interruptions.

Party Wall Agreements

If you live in a terraced or townhouse in London it is likely that you will need to put a party wall agreement in place with your adjacent neighbours. This protects you and your neighbour against issues if there is any damage to their property – as part of the process, a Party Wall Surveyor will take pictures of the wall on both sides to prevent any possible disputes in future. This process can take as little as a few minutes if you have a good relationship with your neighbours – we recommend that you discuss it casually before asking for an official to serve the paperwork as it’s more likely to be agreed quickly.

We work with a highly experienced party wall surveyor, and will provide a referral upon request.