Builders Job Costs

Basement Extension Cost Ranges


As a rule of thumb, when you’re looking to contract and go for your own basement extension, your roundabout average starting cost for an extension will be around £4,000 per square metre. That includes everything. Then you’ll need to add VAT onto this.

This, of course, depends on many variable factors, including how difficult the project is going to be to carry out, how quickly you want the work done, and what kind of services need to be carried around relating to this project. For example, if you want an extension in an expensive, difficult part of London, you may be looking at starting costs of £6,000.

In today’s guide, we’re going to dive into what makes basement extensions, why they cost what they do, as well as detailing everything you need to know when it comes to organising a new basement for yourself.

Why are Basement Extensions So Costly?

Basement cost concept

The reason why you might be a little blown away by the cost of an extension is the fact there are so many processes that need to happen to turn it into a living space. There are also building regulations that need to be met.

It’s worth remembering that if you have an existing cellar, even if it’s unliveable right now, it can be much more cost-effective per square metre to upgrade it and work on certain areas, turning it into a livable basement conversion.

However, whatever kind of basement conversion you’re working on, here are some of the processes and factors you’ll need to think about when it comes to cost and planning.

Ground Excavations

Of course, to extend a basement, you’re going to need to excavate the ground, and this is going to be one of the most costly aspects of the job. This part includes digging, setting foundations, and removing the dug ground off-site.

Any Temporary Support Work

While working on a basement conversion, you’re probably going to need to support any building structure that exists with temporary supports. These can cost a lot, especially when you’re dealing with a lot of square metre spaces.

Waterproofing Your Space

Waterproofing basement

Once the main area is ready, it’s time to work on waterproofing – obviously, you don’t want water getting in here, there, and everywhere. This could be done with concrete, special materials (like a waterproof membrane), or lining the outside of the basement extension with pumps.

You’ll need to figure out the height of the water table and how the water table in your area can change over time to ensure you’re creating a basement that can survive even the worse conditions.

Any Structural Work

Your basement walls need to be able to withstand and hold the load of the soil on the outside, which means structural walls need to be built. These will be the outside of the new basement extension.

Plumbing and Lighting Work

Basement Pipes

Plumbing can be complicated, depending on the existing network that exists – or doesn’t exist. You need to remember that basement plumbing tends to be a bit more complicated than traditional plumbing.

Regarding lighting, since basements are underground or slightly above in areas, good lighting is essential. Always consider what you need in relation to the use of the basement extension space. Natural light is always recommended.

Any External Additions

There’s a wealth of external factors that can be considered, including raising the space up to ground level, creating a well space, roof lights, openable windows, and any mechanical operations you may have.

Access and Ventilation

Think about how you’re going to get into your basement space. Are stairs going to be installed within the space, or will there be a lift or some kind? Maybe you want to use an external access point. You may also need to think about fire escapes.

In terms of ventilation, this must be considered. You may need to add mechanical ventilation if openable windows or roof areas aren’t enough, which can increase the cost of an extension.

Use of Basement Space

Basement laundry room

Finally, once the basement space is up and running, you need to make sure you’re thinking about what kind of room you’re going to be using your basement for, and how much installing the necessary aspects are.

Common uses include a swimming pool, an indoor cinema, and more, all of which need other electrical and plumbing installations to function correctly.

What Consultancy Costs are there for Basement Extensions?

On top of the cost aspects of your basement we’ve listed above, you’re going to want a professional to help the project run smoothly. They will help you overcome any issues you run into, as well as designing and overseeing the project.

You may want to do these jobs yourself, but the experience of a professional can be incredibly beneficial when it comes to ensuring that things are going right.

Typically, a professional consultant will cost around 12-18% of the cost of extension projects. What’s more, you’ll need to consider that a circa 0.5% will be needed, on average, for building regulation fees for your basement extension, and a further 0.5% is needed for party wall agreements, on average.

Party wall agreements will be a contractual agreement that says your neighbours are happy to have you digging around, and potentially next to their walls.

Don’t forget, on top of these basement conversion project prices, you’ll also need to be pay VAT costs and submit a planning application at the beginning. This will averagely set you back around £206, but check your local council to find this out.

What Issues May You Come Across?

Unfinished Basement

Some other basement conversion project issues that may add extra costs to your total can be annoying, but sometimes they aren’t avoidable.

For example, you may need to pay the cost for a construction traffic management plan if you’re going to be disrupting roads and local traffic networks. This is common if you’re working on a building in a built-up area.

You may have noise and acoustic issues to sort out, and you may only be able to work during set hours. If basement extension projects go on for a longer time, you may have to pay increased labour costs.

Other cost factors include vibration reports and dust and air quality reports, specifically during the excavation. This can impact neighbours and those around you.