Basement conversion cost: price guide

If you’re looking to extend your home but can’t extend up or out, have you considered extending down into your basement? Basement conversions are growing in popularity, especially in London, and they’re a great way to add extra space to your home. But how much will it cost?

When you’re thinking about a basement conversion, there are two things to bear in mind: will you be converting an existing cellar or digging and excavating a new one? To convert an existing basement, expect it to cost around £1,400 per square metre. If you need to excavate and underpin a new cellar, it will set you back approximately £2,280 per square metre.

So what factors do you need to consider when you’re planning the cost of your basement conversion? We’ll break everything down in this guide. 

1. Benefits of a basement conversion

2. Will I need planning permission?

3. Supply costs of a basement conversion

4. Waterproofing options for your basement

5. Basement sump pump costs

Benefits of a basement conversion

There are so many benefits of a builder carrying out a basement conversion. Although there’s a large upfront cost, it’s soon offset by the advantages of having your cellar converted. 

Increase your property’s value

Basements are a sought-after feature, especially in London. If you make one or convert yours, your home will become much more attractive to buyers. You can expect a return on investment of up to 1.3%.

Add an extra bedroom

If your family is growing or your children are getting older, they might start wanting some more privacy. A basement conversion works well as an extra bedroom or annexe for teenagers or older family members.

Add an office or gym

Bedrooms aren’t the only options for cellar conversions. If you’re working from home more than before the pandemic, an office might be the best use of the space, but if you’re trying to get fit you could use the room as a home gym. Other popular options include a cinema room or a laundry room.

Alternative to moving

Moving is expensive, and it might be cheaper to stay in your current home but convert the basement. You’ll avoid the hassle and expense of moving but you’ll still get a new space that you love.

Extra storage space

There are lots of ways you can use your cellar conversion to reduce the amount of clutter in your home. Use it as a utility room, a walk-in wardrobe or even a drinks cellar.

Alternative to other extensions

If you don’t have a big enough garden or your home is already extended as far as it can be, a basement conversion is a perfect alternative. They can also be a good option if you’re not able to change the outside of your property since all the work will happen underground.

Will I need planning permission?

It’s unlikely that you’ll need planning permission for your basement conversion as you’re simply converting it into a living space. However, if you plan to add a lightwell or change the usage, then you will need to seek planning permission. You’ll also need planning permission if you’re significantly changing the usage or the basement is a separate unit.

Also, basement excavations are now under permitted development rights, which means you don't need planning permission. However, if you’re having major works or the basement will alter the appearance of your home, it’s worth making sure that you’re covered under permitted development rights. You must also ensure that you’re only digging down one storey and it doesn’t go beyond 3 metres from the back wall of your home. You must also make sure that you leave at least 7 metres between the end of your basement and the boundary of any neighbour opposite.

Listed buildings

It’s worth remembering that listed buildings are dealt with differently. If your property is listed, you’ll require listed building consent if you want to make any internal or external changes.  

Supply costs of a basement conversion

While converting a basement will cost an average of £28,000 for a 20 square metre basement and around £46,000 to excavate one of the same size, there are other costs to think about. For example, if you want to install a new kitchen, this could cost between £4,000 - £8,000. Fitting a staircase could cost £3,000-£5,000, while waterproofing could cost between £1,000-£1,800 per square metre.

Here’s a breakdown of the costs associated with a basement conversion:

Waterproofing options for your basement

Waterproofing is vital to any basement conversion. Since a basement is underground, it’s prone to damp and damage due to weather conditions. You must take all the different types of waterproofing into account and see which one will suit your property best. You should also consider a sump pump to remove water should your basement become flooded.

Floor sealing and tanking

You’ll need to seal your floors as well as the walls. With floor sealing and tanking, you can waterproof the walls and floors at the same time. It involves adding a layer of cement, membrane or bitumen slurry coat which waterproofs the walls and floor. This will set you back between £1,350-£1,650.

Cavity drainage membrane (CDM)

A cavity drainage membrane, or CDM, lines the walls and has a pump system with a studded plastic cavity. Water flows down the CDM and directs towards internal drainage channels before being transported away. It costs anywhere between £1,200 and £1,800.

Barrier system

A barrier system, as opposed to a CDM, holds back water rather than directing it into a drainage system. It works in conjunction with a tanking system that is placed on top of the basement’s original walls and floors. This costs an average of £1,000-£1,500.

Basement sump pump costs

If your basement is below the water table, you’ll want to consider getting a sump pump. When it rains or there are high water table levels, your basement’s waterproofing system’s perimeter drains will funnel the water into the sump basin. The pump will then remove the water.

There are a number of sump pumps on the market:

Get a range of quotes

There’s a lot to think about with a basement conversion, so you want to make sure you’re in good hands. Make sure you get quotes from a number of builders to ensure you get the most competitive price. 

Post a job on HaMuch to get quotes from reliable tradespeople today.

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