Underpinning Certificate Explained

Underpinning is one of the most significant pieces of structural work that can be carried out on a residential property. When a house has been underpinned — whether to address subsidence, to extend a basement, or to increase load-bearing capacity — buyers, solicitors, and mortgage lenders will all want formal evidence that the work was done correctly. That evidence is what practitioners typically call the underpinning certificate.

This guide explains exactly what an underpinning certificate is, who produces the various documents that make it up, why they matter during conveyancing, and what to do if you cannot find the paperwork when you come to sell. It is closely related to the broader topic of selling a house with a history of subsidence, which covers the commercial and practical aspects of marketing such a property.

What is underpinning?

Underpinning is the process of strengthening or deepening the foundations of an existing building. It is most commonly required when:

• The original foundations were inadequate for the soil conditions and the building has suffered subsidence — downward movement caused by the ground beneath the foundations shrinking, washing away, or losing bearing capacity
• A basement or cellar is being created or extended beneath an existing structure
• An extension or additional storey is being added that increases the load on foundations that were not designed to carry it
• Ground conditions have changed due to nearby construction, tree root activity, or changes in the water table

There are several methods of underpinning, including mass concrete underpinning (the traditional “pit and fill” method), beam and base underpinning, and more modern techniques such as mini-piled underpinning and resin injection. The method used will depend on the ground conditions, the cause of the movement, and the structural requirements of the building.

Regardless of the method, all underpinning requires building regulations approval and is classified as notifiable work under the Building Regulations 2010. A structural engineer must design the scheme and supervise the works, and building control must inspect and sign off the completed work.

What does an underpinning certificate consist of?

There is no single official document called an “underpinning certificate” issued by one authority. When solicitors, insurers, and lenders refer to an underpinning certificate, they mean the full package of documentation that evidences the work. This typically includes:

1. The structural engineer's specification and drawings

Before underpinning begins, the appointed structural engineer produces a design specification and detailed drawings for the proposed works. These set out the depth and method of underpinning, the concrete mix and reinforcement details, the sequence of construction, and the monitoring and inspection requirements. This documentation shows that the works were properly planned by a qualified professional.

2. Site investigation or ground survey report

A ground investigation is usually carried out before underpinning is designed. This may include trial pits, borehole surveys, or laboratory analysis of soil samples. The results inform the engineer's design. Buyers and lenders want to see this because it confirms that the underpinning was designed with full knowledge of the actual ground conditions, rather than as a “best guess”.

3. The structural engineer's completion certificate or sign-off report

Once the underpinning is complete, the structural engineer visits the site to confirm that the works have been carried out in accordance with the specification. They issue a completion report or sign-off certificate confirming this. This is the engineer's professional statement that the work meets the design requirements and, in their opinion, has addressed the cause of the problem.

4. Building regulations completion certificate

Separately from the engineer's sign-off, the local authority building control (LABC) or an approved inspector must issue a building regulations completion certificate confirming that the underpinning work has been inspected and found to comply with the Building Regulations 2010. This is the official regulatory sign-off and is a key document that buyer solicitors will request. Without it, the underpinning is not formally compliant from a building regulations perspective. See our guide on what to do when building regulations sign-off is missing for more detail on this specific issue.

5. Monitoring records (where applicable)

For subsidence-related underpinning, the engineer may require a period of settlement monitoring before and after the works to confirm that movement has ceased. Monitoring is typically done using “tell-tales” across cracks or precision levelling of datum points. The monitoring records, and the engineer's interpretation of them, form part of the full documentation. A final monitoring report confirming no ongoing movement is particularly valuable for buyers and their lenders.

6. Contractor's guarantee (where provided)

Some underpinning contractors provide a 10-year structural guarantee for their work. This is a warranty from the contractor (and sometimes backed by an insurance-backed guarantee scheme) confirming that the underpinning will remain effective for the stated period. Not all underpinning work comes with such a guarantee — it depends on the contractor and the date of the work — but if one exists, it should be included in the documentation pack.

Why buyers and lenders require underpinning documentation

From a buyer's perspective, discovering that a property has been underpinned raises two related concerns: whether the underlying problem has been fully resolved, and whether the property carries ongoing structural risk. The underpinning documentation addresses both.

Mortgage lenders have their own reasons to scrutinise previously underpinned properties closely. A property that has suffered subsidence and been underpinned is considered a higher lending risk than one with no such history. If the underpinning fails or the property suffers further movement, the lender's security (the property itself) is impaired. Most mainstream lenders therefore require:

• The full underpinning documentation package described above
• Confirmation from a current structural engineer that there is no evidence of ongoing movement (this may require a fresh inspection even if the original underpinning was years ago)
• Buildings insurance from a specialist insurer who is prepared to cover the property given its history

Some lenders will not lend on previously underpinned properties at all, regardless of the documentation available. This significantly restricts the pool of mortgage-dependent buyers who can purchase your property. Cash buyers are not subject to lender requirements, but their solicitors will still require the documentation for due diligence purposes.

For a detailed account of how subsidence history affects the sale process more broadly, including pricing and buyer negotiations, see our guide on selling a house with foundation issues.

The TA6 form and disclosure obligations

When selling a property, you are required to complete the TA6 Property Information Form, which asks specific questions about the property's history. The TA6 asks whether the property has ever been affected by subsidence, heave, or landslip, and whether any underpinning or other structural remediation has been carried out.

You must answer these questions honestly and accurately. Failure to disclose known underpinning could amount to misrepresentation, which can expose you to claims from the buyer after completion. The relevant case law makes clear that sellers cannot simply choose not to mention significant structural works they are aware of.

If you are uncertain whether work carried out at the property constitutes underpinning (as opposed to, for example, repairs to a cracked foundation or repointing of brickwork), you should obtain a structural engineer's opinion before completing the TA6 form.

What if the underpinning documentation is missing?

Missing documentation for underpinning is more problematic than missing paperwork for many other types of building work, because standard indemnity insurance policies are not designed to cover it. The absence of an underpinning completion certificate cannot simply be resolved by taking out an off-the-shelf indemnity policy in the way that missing building regulations sign-off for a loft conversion often can be.

There are several steps to take if you cannot locate the documentation:

Step 1: Search available archives

Contact your local authority building control department and ask them to search their records for a building regulations application and completion certificate relating to underpinning at your property. Even old records are often retained, though the ease of retrieval varies by council and by the age of the records. Allow two to four weeks for this search.

Also try to identify the structural engineer who designed and supervised the work. If you can trace them from any documents you do have (estate agents' particulars when you bought, previous solicitor correspondence, or the names of any contractors involved), contact them directly. Their practice may still hold the drawings, calculations, and inspection reports in their archive.

Step 2: Instruct a current structural engineer

If the original documentation cannot be retrieved, the next step is to instruct an independent structural engineer to carry out a current inspection. They will assess the condition of the underpinning (to the extent that it is visible or can be investigated without destructive opening-up), review the property for any signs of ongoing movement, and produce a report on their findings. This is explored further in our guide on structural engineer reports when selling.

A positive report from a current engineer — confirming no evidence of ongoing movement and no visible defects in the underpinning — will go some way towards reassuring buyers and their solicitors, even without the original documentation. However, it does not substitute for the building regulations completion certificate in the eyes of most mortgage lenders.

Step 3: Seek specialist indemnity insurance

Specialist legal indemnity insurers can sometimes provide cover for missing underpinning documentation, but this is a more complex product than standard building regulations indemnity insurance. The insurer will want to see a current structural engineer's report confirming no ongoing movement, and the premium will reflect the specific circumstances of the property. Not all lenders will accept such a policy in lieu of the original documentation, so your solicitor must check the buyer's lender's specific requirements before relying on this route.

Step 4: Regularisation (where possible)

If the building regulations completion certificate is missing but the work was carried out after 1985, it may be possible in principle to apply to the local authority for retrospective regularisation. However, because underpinning involves buried foundations, the local authority surveyor will not be able to inspect the work in the normal way without substantial investigation works. In practice, regularisation for underpinning is rarely the most practical route where the original documentation is simply lost rather than never having existed.

Buildings insurance and previously underpinned properties

Buildings insurance for a previously underpinned property is a specialist matter that both you as seller and any buyer need to address. Standard home insurance policies typically:

• Exclude subsidence damage where the property has a known history of subsidence, or apply a very high excess (commonly £1,000 or more) for any subsidence-related claim
• Require full disclosure of any history of subsidence, heave, or underpinning on the proposal form — failing to disclose this can invalidate the policy
• Reserve the right to decline cover or exclude subsidence from the policy scope

Specialist insurers who deal with non-standard properties can usually provide buildings insurance cover for previously underpinned properties, albeit at a higher premium than a standard policy. Your buyer should be made aware of this before exchange so they can obtain specialist insurance quotes in advance and factor this into their decision.

As a seller, it is worth noting that you should continue to maintain buildings insurance on the property until completion, and your current insurer should have been informed of the underpinning history when you first took out your policy. If they were not, there is a risk that your current policy is not valid.

Selling a house with structural cracks: the relationship to underpinning

Not every property that has experienced cracking has been underpinned, and not every property that has been underpinned shows visible cracking after the works. However, the two subjects are closely linked. Buyers who notice cracking during a viewing — particularly diagonal cracks at window and door reveals, or stepped cracking in brickwork — will want to know whether the cause was ever investigated and whether underpinning was carried out.

If your property has structural cracking but has not been underpinned, you may need a structural engineer to assess whether underpinning is necessary before you sell, or at least to confirm that the cracking is historic and not indicative of ongoing movement. Our guide on selling a house with structural cracks covers this scenario in more detail.

Summary: what sellers need to have in order

For broader context on the documents you will need to assemble before selling, see our guide on what a property certificate pack contains. Starting to gather documentation well before you list — and identifying any gaps early — gives you the best chance of a smooth sale.

Sources

• Building Regulations 2010 (SI 2010/2214) — legislation.gov.uk
• Building Act 1984 — legislation.gov.uk
• Institution of Structural Engineers (IStructE) — istructe.org
• Local Authority Building Control (LABC) — labc.co.uk
• UK Finance Mortgage Lenders' Handbook — ukfinance.org.uk
• Association of British Insurers (ABI): Guidance on subsidence and buildings insurance — abi.org.uk
• Law Society TA6 Property Information Form (5th edition) — lawsociety.org.uk
• Misrepresentation Act 1967 — legislation.gov.uk

Frequently asked questions