Basement Extensions

Basement conversions are becoming increasingly popular in the UK. In 2012, there were 1,084 planning applications in England, Scotland and Wales; in 2016 there were three times that number. And why not? You get more living space without having to extend outwards or move, which is particularly useful if you live in the city. Before you get started though, let’s take a look at some of the common pitfalls when carrying out a basement extension or conversion.


There's a lot of water under the ground, and it can cause huge problems if you haven't done the necessary site investigations beforehand, including a soil survey to determine how saturated the earth is. When we say huge problems, we mean that one possible worst case scenario is that the water pressure literally lifts the basement out of the ground. Less alarmingly, it might just leak.

Modern basements usually use something called a cavity membrane to keep the water out – a plastic covering around the whole basement which keeps the water out and runs it instead into a sump where it can be pumped out. You'll need to have this system maintained regularly, and the pump should have a battery backup. Alternatively, you can apply a cement-like waterproof coating around the outside of the basement, known as tanking.

Like most homebuilding and home improvement issues, the risk of problems can be greatly reduced by using reputable architects and builders, for whose work you can obtain an insurance-backed guarantee.


Your neighbours might be unhappy – either because they're worried about damage to their home, or because they're unhappy about the noise, dirt and disruption that comes with the work. You need to give your neighbours at least two months' notice of the work; they can't stop you doing it, but they have a say over how and when it's done. If they're not happy you will probably have to resort to a party wall award – a document drawn up by surveyors detailing what the work will involve, which is likely to cost at least several hundred pounds.

You don't always need planning permission for a basement conversion, but if you do need to submit a planning application, we would always advise making sure your neighbours are happy with the plans first.

Structural issues

Structural issues with basements are rare, but can be very severe if they occur. If the basement ceiling isn't high enough for you to use it comfortably, the foundations may have to be underpinned (made deeper) and the floor of the basement lowered; some joists may need to be reinforced too. You should always consult a structural engineer, and structural work will also need approval from a council building control inspector.


Despite all that water, you may also need to have a means of escape from the basement in case of fire – a door or window that you can get out of, or fireproof stairs. Even if you're not legally required to have one, you might want to think having one installed regardless.

If you're thinking about a basement extension or conversion, Bluelime Home Design can help out. We offer architectural services in Kent and London, ranging from design consultation to planning applications. Contact our BexleyBromleyErithDartford or Croydon office today.

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