Basement Conversion

A basement conversion is an increasingly popular way of maximising space in a property. You own the ground below you so make the most of it. A basement conversion is not cheap as we’ll show you later, but they are wonderful, and here is a simple one we converted in Bath.

this basement conversion is used as a small home cinema and laundry

Basement conversion costs

Let’s start with the costs. Our first question we ask clients who contact us for a basement conversion is to ask what budget they have available. It’s important to be realistic.

The key drivers for a basement conversion, just to create a habitable space are:

  • the volume of excavation
  • the degree of underpinning
  • the challenges posed by site conditions (water table, contractor access and so on)
  • access required to the basement
  • drainage arrangements

The fittings, furnishing, swimming pools and so on are then extra, or course.

The rules of thumb that we use for Bath and Bristol are as follows (all plus VAT);  there will be regional variations:

  • To convert an existing cellar to a habitable space wth “minimal” alterations: £1000 – £1300 /m2
  • To lower the floor to create a head clearance of over 2m: £1650 – £2200 /m2
  • To dig out from scratch: £3000 – £4000 /m2

Other key costs:

  • To create a lightwell:  £5000 – £7000
  • To create external access eg for steps:  £5000 – £7000
  • Fees (structural engineer, designer / architect, party wall surveyor if required): £3500 – £4500

Basement conversion quotations

Here are some case study figures that we have priced (uplifted to 2017 economic conditions assuming 3.5% annual inflation rate):

15 m2 in Bath: full excavation, underpinning, external access, two side windows, tanking, concrete floor over wet underfloor heating, basic downlighters:  £60k + VAT

80 m2 in  Bath: part dig out / part existing, part tanking, basic heating and lighting : £175k + VAT

30 m2 in Bristol: part excavation to one side (other side at full height), part tanking, basic heating and lighting: £70k + VAT

32 m2 in Bath: part dig out / part existing, part tanking, creation of 3 separate rooms, basic heating and lighting : £87.5k + VAT

We would need to include prices for any additional complications such as drainage and pumping on a csae by case basis, but hopefully the figures above give you a ballpark to work from.

basement conversion uses

Given the expense of a basement conversion, you ought to have a good idea of what it will be used for. We installed a rudimentary home cinema in one, and a laundry. An area for teenagers is a sound prospect, giving them space with their friends away from the main family living room. A games room works wel as does a luxury seating living space. In fact, anything is possible, but be clear on your intentions and what will happen to the remainder of the house with rooms perhaps no longer in regular use.

laundry in basement conversion

Basement storage

To keep costs down, you could excavate around the edge of the basement conversion to waist height.  Here you can create storage cupboards at a useful level.  Fit them with proper floor such as concrete, and install lighting so you can see. In some examples you will be losing a large storage area so see where you can reinstate sufficient storage for your needs. We’ve fitted boilers, water softeners and IT hubs in basements so they offer scope to provide more storage elecwhere in the home.

Basement lighting

Try to pass as much natural light as possible into the basement room;  this can be from a light well, ground level windows or borrowed from adjacent rooms. Some designs incorporate a large glass panel in the floor above for the basement to borrow from.

We tend to use recessed LED downlighters in warm white spaced with 900mm spacing. However, there are other options;  recently we advised on peripheral lighting where striplights are fitted behind panels in the walls or ceiling yet provide cosy atmospheric lighting. Whatever your choice, use several circuits for fleibility. We’ve written a separate guidance page on choosing home lighting.

Basement Conversion

This project cost around £60 000 + VAT and delivered a space for teenagers, a gaming area, surround sound cinemas system and a laundry. It also incorporated plenty of storage for games and camping equipment as well as the new condensing combi-boiler and water softener.

step lights

The property had an undercroft, but still required significant excavation. The structural engineer determined the degree of underpinnng and steels required to support the house. In fact this basement excavation was part of a larger project in which we installed an open plan kitchen diner in a glass extension.

side return extension to Edwardian end of terrace

You can see the steps to the basement on the left. We’ve written separately about this glass side return extension.

Because we designed this as a total project, we decided that the steps sat best within the glass extension.  This area was large enough, whereas the basement would have lost valuable space within internal access.

Excavation naturally went deeper than the finished floor as there was significant additional insulation to install.

basement excavation

The walls were underpinned and waterproofed with a cavity drain membrane. This permits water to penetrate from the ground, but then trickles down the membrane to a channel which transfers it to a sump. In the photo below you can see the underpinning through the tanking, supporting the original house walls.

basement tanking

We installed the wet underfloor heating pipes before the screed and concrete slab. We selected chippings to add to the top 25mm of concrete which then showed through when it was polished.

polished concrete floor

This basement conversion now provides a haven for teenagers and a place to do the laundry;  all away from the main living areas of the rest of the house.