If you need more space, adding a below ground level can be a useful route to gaining extra space, particularly if you do not have the possibility to extend above ground or do not want to lose precious outdoor areas. This article on basement extension gives advice on maximising your home’s potential.

Firstly, you should start with checking whether you could be successful in gaining planning permission for a basement, rather diving straight into the technical aspects of a basement.

Check to see if there are any constraints on your home regarding a basement development, such as it being listed or in a conservation area. These issues could affect what you can do regarding a basement development, and the permissions you will need to obtain.’ Another thing to do is to check if any of your neighbours have carried out a basement project that you could use as a planning precedent.

Assessing the site

For the construction of the basement, one of the limiting factors that can affect your plans is the presence of water. The presence of a high water table could increase the costs. It is therefore essential to check the Environment Agency’s flood map to find out if your home is in a flood zone.

The access to the construction site needs to be checked, as it  is a common issue that will have an effect on the build and the cost of the build. If you live in the middle of a terrace block, excavating beneath your home and transporting the soil away from the site will present more of a logistical challenge than a semi-detached or detached house. However, there are ways of solving this with an experienced contractor, but it needs to be identified early to allow for the additional costs in the budget.

As part of the design process, your architect or you, the Client, will need to appoint a structural engineer experienced in basement design. There are several investigations and reports that will need to be done to give the required information for the basement design and drawings, such as a soil investigation and report to determine the type of soil and what the ground water level is for example and the flood risk. Additional site investigations will be required on site, including digging trial pits, to establish what the existing foundations are and their depth etc. Most local authorities are also now insisting on Basement Impact Assessment reports for proposed basement projects.

Some houses already have a cellar, so it may be possible to convert this into usable living space if the headroom is adequate and there is the potential for allowing natural light into the space. The cellar will need to be either tanked internally or it can be lined with an internal drainage system that drains the water to a sump and it is pumped out when required, which is usually the most popular and preferred solution.

Planning permission

In most London boroughs, basement projects now automatically require planning consent, although in the past some were carried out under permitted development rights.

It is very important to check your Local Council’s basement policies, to make sure these are understood from the start, as they could have a significant impact on your plans for a basement.

Obtaining permission will depend largely on the depth of the basement, the layout and whether any changes are being made to the exterior of the property. Building a light well to allow light into the lower level automatically requires formal consent.

It is advisable to appoint an architect experienced in basement developments to help you gain permission for the basement development, as there are several factors that need to be addressed.

Your programme should allow enough time to allow for any issues or delays that may occur during the planning phase. Basements are more complex projects than normal straightforward rear or side extensions or loft conversions, so it is essential to engage the right team of experts from the outset.

Assembling the best team

Basements are more complex projects than straight forward rear and side extensions or loft conversions, so it’s vital to engage the right team of experienced experts from the outset. This will pay dividends throughout the project and help you achieve a successful project.

You should engage an Architect with experience in this type of build to give your proposed basement project the best chance of success. The Architect will give you advice when it comes to choosing and appointing a suitably experienced and qualified structural engineer to carry out the necessary surveys and do the required structural design of the basement. An architect will be able to address key factors, such as how the basement connects to the rest of the living space and how to maximise light.

How long does a basement extension take?

Most basements are below an existing property, and in order to construct it, it is usual for the ground floor to be removed so the construction team can excavate beneath. The normal way that the basement is formed below an existing house is by the traditional underpinning method, which is quite a slow process and may impact on other parts of the house, as you have to underpin the existing walls with concrete in a defined sequence.

Basement projects can take longer than normally expected, often up to a year or more. However, the conversion of a cellar to a basement included in the refurbishment of a house could take approximately six months. Some homeowners do stay in the property while the basement work is done, however, this is generally not recommended as it will likely take longer to construct the basement and will cost more.

Sometimes the neighbours may experience some disruption due to your basement construction, but often this is not as much as they might fear. The neighbours may be apprehensive as they would have read about some basement horror stories in the press or heard some stories from friends, which is why it is important to tell your neighbours about your plans from the beginning and explain that you will be employing an experienced team of Professionals for the project. You do not want them to first learn about your proposed basement plans when they receive the  official planning notification from the council, as this will not help with your neighbourly relations as communication with them is key throughout your project.

How much does a basement extension cost?

There are a number of factors that will affect the cost of your proposed basement development, such as the access to the site, the size of the scheme and the quality of the fit-out amongst other factors.

The current cost estimate would be in the region of £4,000-£5,000 per square metre, depending on the specification and level of finishes used, plus fees and VAT. If you already have an existing  cellar and it has sufficient headroom, it could be waterproofed and fitted out for approximately half this amount.  You would also need to factor in that professional fees could add approximately 20% to these costs. You will also need to add in a contingency amount of approximately 10% to 15% to allow for unforeseen issues, as digging below ground does have its risks, which cannot be predicted.