Published · By Brixter Construction
If someone has mentioned “underpinning”, you’ll want to know what it involves and whether you really need it. This guide explains the basics for homeowners in Hassocks, Burgess Hill, Haywards Heath and nearby.
General guidance only: decisions about foundations should be based on an assessment by a structural engineer or surveyor.
What is underpinning?
Underpinning strengthens the foundations of an existing building, usually by taking them deeper so they rest on firmer ground or can carry more load. The Planning Portal describes it as digging out the soil beneath the existing foundation and replacing it with foundation material, normally concrete, in phases, so the house stays supported throughout.
When is underpinning needed?
The Planning Portal gives two main reasons:
- The existing foundations have moved. This is usually caused by poor soil or a change in ground conditions, such as subsidence.
- The building is about to carry more load. For example, an extra storey above ground or a new level below ground, where the existing foundations aren’t deep enough.
Signs worth getting checked
- New cracks, or cracks that are getting wider, especially diagonal cracks near doors and windows
- Doors and windows that start to stick when they didn’t before
- Walls or floors that look out of line
These don’t automatically mean subsidence. Cracks often have simpler causes, so the first step is always an assessment.
Underpinning isn’t always the answer
Movement often has a fixable cause, such as a leaking drain or tree roots drying out the soil. Dealing with the cause comes first and is sometimes enough, and an engineer may monitor cracks for a while before deciding. If you suspect subsidence, contact your buildings insurer early.
How underpinning is usually carried out
Each project follows the engineer’s design, but the general process looks like this:
- Investigation. The Planning Portal notes that the first step is usually a trial hole dug next to the existing footings, so that a structural engineer or surveyor can see what the foundation sits on and how deep it goes.
- Design. The engineer decides the method and sequence. Common methods include:
- Mass concrete: short sections (“bays”) are dug and filled with concrete one at a time, in a set order.
- Beam and base: a reinforced concrete beam spreads the wall’s load onto concrete bases.
- Mini-piled: small piles carry the load down to deeper, firmer ground.
- Approvals and notices (see below), before work starts on site.
- Excavation and concrete in sections, with building control inspections at the stages they ask for.
- Packing and making good, then reinstating ground, paths and finishes.
The Planning Portal also highlights taking care with nearby sewers and drains.
Building Regulations
Underpinning is defined as “building work” under the Building Regulations, so approval is normally needed. According to the Planning Portal, approval usually involves preparing a structural design for the underpinning, including how the work will be carried out.
You can apply to your council’s building control team or to a private registered building control approver. GOV.UK describes two routes: full plans, checked before work starts, or a building notice, where work can start two days after submitting. Mid Sussex District Council lists underpinning among the work suitable for a building notice. A building notice can’t be used for every job, though (for example, some work close to or over a public sewer needs full plans), so check with building control which route suits yours.
At the end you should receive a completion certificate. GOV.UK warns that without the right certificates you may have problems when you come to sell your home, so keep it with your house papers.
Party Wall etc. Act 1996
If your house shares a wall with a neighbour (most terraces and semis do), or the digging is close to a neighbour’s building, the Party Wall etc. Act 1996 may apply. The government’s explanatory booklet covers this in detail. In summary:
- Underpinning the whole thickness of a party wall is work under section 2 of the Act. You must give the adjoining owner at least two months’ notice.
- Excavating within 3 metres of a neighbour’s building to a depth below their foundations, or within 6 metres where the dig crosses a 45° line down from their foundations, needs at least one month’s notice under section 6.
- If your neighbour doesn’t agree in writing within 14 days, a dispute is treated as having arisen, and a party wall surveyor (or one for each owner) will draw up an “award” setting out how the work is done.
The Act is separate from Building Regulations, so you may need both.
Planning permission
Underpinning alone rarely changes how a home looks, but if your property is listed or in a conservation area, or the work is part of a bigger project, check with your local planning authority.
Underpinning as part of a bigger project
Underpinning is often one stage of a larger job: strengthening foundations before an extension, or alongside load-bearing wall removal as part of wider structural works. If you are planning to open up your home as well, our steel beam guide explains how that work is approached.
Thinking about underpinning in Mid Sussex?
Brixter Construction is a building company based on Keymer Road in Hassocks. We carry out underpinning and structural work across Mid Sussex, Brighton & Hove and Lewes, including underpinning in Burgess Hill. We work to your engineer’s design and alongside building control and any party wall arrangements.
Get a free, no-obligation quote
Call Josh on 07518 403458 (Mon–Fri, 7:30am–6pm) or send us your details through our contact form. Quotes are free and without obligation.
Related pages
- Underpinning
- Structural works
- Underpinning in Burgess Hill
- Removing a load-bearing wall: a steel beam guide for Mid Sussex homes
- All guides
Sources
Official guidance checked 1 November 2026:
- Planning Portal: Is building regulations approval needed for underpinning?
- GOV.UK: Building regulations approval – how to apply
- GOV.UK: Building regulations approval – when you need approval
- Mid Sussex District Council: Make a Building Control Application
- GOV.UK: The Party Wall etc. Act 1996 – explanatory booklet