Survey finding
Tanking failure in a basement: what it means and what to do
Tanking failure in a basement or cellar is a costly but contained finding. This page covers Type A/B/C waterproofing, what fixing it costs, and how lender and insurer markets treat it.
Last updated: 6 May 2026. Editorially reviewed: 20 May 2026.
Finding
Tanking failure in basement
What this usually means
Tanking is a waterproof barrier applied to basement walls and floors to keep groundwater out. Most UK tanking is either Type A (cementitious render) or Type C (cavity drained membrane). Failure shows as efflorescence, damp patches, blistering paint, or visible water ingress at the foot of walls. The cause is usually age, original installation defect, or hydrostatic pressure exceeding the tanking's design limit.
Why it matters
Basement tanking failure is rarely a structural risk to the property but is expensive to remediate and disruptive, often requiring full strip-out and re-tanking. BS 8102 sets out the design standards. Lenders rarely refuse but may impose a retention if remediation is required before basement use.
Ask your surveyor
- Check:What type of tanking is in place, A (cementitious), B (structurally integral), or C (cavity drained)?
- Check:Where are the visible failure points and what is the most likely cause?
Ask the seller
- Check:When was the tanking installed and which system?
- Check:Are there any guarantees from the original installer?
Next steps
- •Get two written quotes from local trades before negotiating with the seller.
- •Speak to your mortgage broker before exchanging if the finding affects mortgageability.
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What you need to know
Severity
Significant. Specialist follow-up usually warranted before exchange.
Typical cost to fix
Specialist damp-proofing inspection £150–£400. Patch tanking repair: £600–£1,800. Full re-tanking of a typical UK basement: £8,000–£25,000 depending on size and Type A/B/C system specified. Cavity drained membrane systems with a sump pump add £2,000–£5,000.
Mortgage impact
Most lenders accept tanking findings if remediation is quoted and the basement is not the only habitable space. Basements counted in the property's habitable square footage may trigger a retention until cover is documented.
Insurance impact
Tanking failure is wear-and-tear, not an insured event. The pump and electrics for cavity drained systems are typically the insured items.
When to pull out
Pull out only if the basement is fundamental to the purchase price, full re-tanking cost is high, and the seller refuses to fund or deduct.
When to renegotiate, and by how much
Get a specialist quote for re-tanking to BS 8102 spec. Negotiate on quote plus 15–20% buffer.
Thinking of pulling out or renegotiating? What to do after a bad survey
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Cellar / basement damp , often sits near tanking failure in basement on a survey and is the next thing to check.
How MyPropertyScan produces this guide, and what it is not
Published by MyPropertyScan and written by Ziad Nasr, who is accountable for its accuracy. No page here is written or reviewed by a chartered surveyor, conveyancer or regulated adviser, and we do not claim otherwise.
This page is not:
- Chartered surveying — we do not inspect properties or produce RICS Home Survey reports
- Legal advice — we do not act as conveyancers or advise on title, tenure or contracts
- Mortgage or insurance advice — we are not regulated by the FCA and give no recommendations
Figures and process steps come from the published UK datasets and standards listed on this page, not from professional practice. Pages are updated when source coverage, survey cost assumptions, or lender and legal requirements materially change. Dataset limitations are set out on the methodology page.
Spotted something out of date or wrong? Email hello@mypropertyscan.com and we will correct it and change the update date.
Sources used
We use UK public and specialist sources where they are available. Public datasets can be incomplete, delayed, or missing for some addresses. Treat them as a starting point, not as a replacement for professional advice.
Source standard: preference goes to official government datasets, statutory bodies, professional standards, and primary dataset publishers. We cite the source family on the page and explain coverage limits rather than filling gaps with unsupported estimates.
- Professional standard: RICS Home Survey StandardRICS standard for condition-based residential surveys in the UK.
- Learn more from: RICS consumer guide to surveysConsumer-facing guide to survey types and when each level is appropriate.
- Official guidance: GOV.UK buying or selling your homeGovernment overview of the home buying and selling process.