The short answer
It depends on whether you need to dig down. If you are converting an existing cellar that already has enough head height and you are not lowering the floor below the existing foundations, underpinning is often not required. But the common reason to underpin is to increase ceiling height: to create a comfortable room you frequently lower the basement floor, and once you excavate below the existing footings the walls must be underpinned so they bear on new, deeper foundations and the house stays stable. Underpinning is a structural alteration, so it must be designed by a structural engineer, approved under Part A of the Building Regulations, and — because it usually involves a shared or boundary wall — it typically triggers the Party Wall etc. Act 1996. It is carried out in carefully sequenced short bays so the wall is never undermined along its full length at once. Whether you need it comes down to your target floor level versus your existing foundation depth.
Underpinning is the part of a basement project people worry about most, but it is not always needed — it is driven by how much head height you must gain. The notes below explain when it is triggered and how it is done safely.
Underpinning at a glance
- Triggered bylowering the floor below existing footings
- Not always neededif existing depth gives head height
- Designed bya structural engineer
- RegulationPart A (structure) + Building Control
- Usually also engagesParty Wall etc. Act 1996
When underpinning is triggered
The decisive question is your target finished floor level against the depth of the existing foundations. If the cellar is already deep enough to give habitable head height once you allow for the floor build-up, insulation and any drainage, you may be able to convert without underpinning. If you need to lower the floor to gain head height — which is the usual case in older terraces with shallow cellars — and that takes the new floor below the bottom of the existing footings, the walls lose their support and must be underpinned: new concrete foundations are built beneath them down to a sound bearing level. This is structural work that changes how the whole house is supported, so it is never something to specify by eye. A structural engineer assesses the loads, the ground and the depth required.
How underpinning is done safely
Underpinning a wall to deepen a basement is carried out in a carefully sequenced bay-by-bay (hit-and-miss) pattern. The wall is divided into short numbered bays, and only non-adjacent bays are excavated and concreted at any one time, so the wall is always supported between the open sections and is never undermined along its full length. Once each bay's new foundation has cured and is pinned tight up to the existing wall, the next bays are done, until the whole wall sits on new, deeper foundations. The sequence, bay width and concrete mix are all specified by the structural engineer, and Building Control inspect the excavation, reinforcement and concrete. Done correctly this is a controlled, well-understood operation; done badly — too many bays open at once, or no engineer — it risks movement and cracking, which is why it is a designed and inspected process.
| Step | What happens |
|---|---|
| 1. Design | engineer specifies depth, bays, sequence, reinforcement |
| 2. Bays marked | wall divided into short numbered sections |
| 3. Hit-and-miss dig | only non-adjacent bays open at once |
| 4. Pour and pin | new foundation cast, packed tight to the wall |
| 5. Repeat | remaining bays done in sequence |
| 6. Inspect | Building Control sign off each stage |
Indicative sequence for guidance. Source: structural engineering practice and Building Control requirements.
The approvals underpinning brings with it
Because underpinning is a structural alteration, it pulls in several approvals at once. It must comply with Part A (structure) of the Building Regulations and be checked by Building Control, with a structural engineer's design and calculations behind it. Where the wall being underpinned is a party wall, or where you excavate within 3 metres of a neighbour below their foundation level, the Party Wall etc. Act 1996 applies, so you serve notice, agree an award and record a schedule of condition before work starts. If the basement involves external excavation or a new lightwell, planning permission is usually needed too. The practical message for homeowners is that underpinning is rarely a standalone decision: it sits at the centre of the structural, Building Control and party wall strands of a basement project, and it is the engineer's design that ties them together. A reputable contractor will not start underpinning without that design and the relevant approvals in place.
Frequently asked questions
Can I convert a basement without underpinning?
Yes, if the existing cellar is already deep enough to give habitable head height without lowering the floor below the existing foundations. Underpinning is only triggered when you dig down past the footings to gain height, which a structural engineer can confirm from your levels.
Why is basement underpinning done in small sections?
To keep the wall supported throughout. The work is sequenced in short, non-adjacent bays (hit-and-miss) so the wall is never undermined along its full length at once. Each new foundation is cast and pinned before adjacent bays are excavated, under an engineer's specified sequence.
Does underpinning need a structural engineer and Building Control?
Yes. Underpinning is structural alteration, so it must be designed by a structural engineer, comply with Part A of the Building Regulations and be inspected by Building Control. Where a party wall is involved, the Party Wall etc. Act 1996 also applies.
Sources & further reading
- Planning Portal — Building Regulations Part A (structure)
- GOV.UK — Party Wall etc. Act 1996 booklet
- RICS — foundations and underpinning guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.