Guide
Basement conversion cost: what actually changes the number
Updated
Three very different jobs get sold as a basement conversion. Knowing which one you are buying is worth more than any published rate per square metre.
The three jobs behind one word
- Converting an existing cellar
- The structure stays as it is. Work is waterproofing, damp management, ventilation, drainage, electrics, stairs and finishes. The cheapest by a wide margin, and the only version where you are not doing structural work under an occupied house.
- Lowering an existing cellar floor
- Existing walls are underpinned in a controlled sequence to gain headroom. Now you have temporary works, spoil, engineering design and, if neighbours are close, party wall notices. A structural project with a fit-out attached.
- Digging a new basement
- Excavating a new box under a house that is still standing, or under the garden. Propping, retaining structure, waterproofing designed from scratch, large spoil volumes and the longest programme. The most expensive way to add space to a home in the UK, and materially more per square metre than an equivalent extension above ground.
Where the money actually goes
| Cost line | Convert | Lower floor | Dig new |
|---|---|---|---|
| Structural and temporary works design | Rarely | Yes | Yes, extensive |
| Underpinning or retaining structure | No | Yes | Yes |
| Spoil removal | Minimal | Significant | Dominant on restricted sites |
| Waterproofing design and system | Yes | Yes | Yes |
| Party wall notices and surveyors | Usually not | Often | Almost always in a terrace |
| Building regulations approval | Usually | Yes | Yes |
| Pumped drainage and ventilation | Yes | Yes | Yes |
Why we do not publish a rate per square metre
You will find per square metre figures for basements all over the internet. The trouble is that the same number is used for a dry Victorian cellar being tanked and decorated and for a new reinforced concrete box excavated by hand through a hallway in Zone 2. Those jobs differ by a multiple, not a percentage. Publishing a single figure would give most readers a wrong answer with a confident face on it, so we describe what drives the price instead and let contractors price your actual property.
What makes a quote comparable
- State the method you want priced: convert, lower, or dig new. If you do not know, ask each contractor to state the method they have assumed.
- Fix the finished headroom in the enquiry. Depth is the quiet multiplier on every structural line.
- Describe the access honestly, including whether spoil must travel through the house and whether a lorry can stand outside.
- Ask for the waterproofing system by type and for the name of the waterproofing designer.
- Ask what is excluded, especially structural design fees, party wall surveyor fees, permits, and the fit-out.
- Ask who carries ground risk and whether any site investigation is included.
Verify before you commit: figures for basement work vary so much by region, access and ground that any number you read, here or anywhere, is a starting point for a conversation and not a budget. Get at least three quotes on the same written scope, and treat a quote given without a site visit with real caution.