Hampshire · Subsidence Risk
Subsidence risk in Portsmouth: what to check before buying
Portsmouth sits on Tertiary clays and sands of the Hampshire Basin (London Clay and Bracklesham/Bagshot beds), with made ground and reclaimed land in places around the harbours. These clays are shrink-swell susceptible, though the flat, sea-level setting makes flood the headline issue.
Last updated: 24 August 2026. Editorially reviewed: 20 May 2026.
Subsidence index rank
Portsmouth ranks #8 of 46 in the UK Subsidence Risk Index
- Clay band
- Moderate–High
- Mining significance
- Limited
- Primary driver
- Clay shrink-swell
Portsmouth ranks #8 of 46 in the UK Subsidence Risk Index, sitting in a moderate-to-high shrink-swell susceptibility band on the BGS-derived scale used to build the ranking.
Historic mining has limited documented influence in Portsmouth; the primary driver below is the main factor to watch.
For Portsmouth, clay shrink-swell (ground moving seasonally as clay dries out and rehydrates) is the dominant local driver, in line with the "Clay shrink-swell" classification the index assigns to the area.
See the full 46-city ranking or explore the UK subsidence map. This is a city-wide index, not an address-level result — check the specific property separately before offering.
Tool shortcut
Check subsidence signals for a UK address in 15 seconds
Directional BGS 1:1 million engineering geology, building age, tree context and the things to ask your surveyor.
Run a free previewBGS clay susceptibility for Portsmouth
BGS GeoSure rates clay shrink-swell susceptibility as moderate to high across parts of Portsmouth where Tertiary clays are near surface. Subsidence claim density is meaningful but secondary to coastal-flood exposure.
BGS GeoSure publishes shrink-swell susceptibility ratings at 1:50,000 scale, covering Great Britain as a premium local-level product. Conveyancing environmental searches can combine licensed ground-stability data with mining and contamination layers for property-focused due diligence.
Source boundary: MyPropertyScan's instant scanner does not query parcel-level GeoSure. It uses public BGS Engineering Geology at 1:1 million scale as a directional indicator; obtain GeoSure or a conveyancing search separately.
Trees, drainage and other risk factors in Portsmouth
Trees on the Tertiary clays in proximity to foundations are the main shrink-swell trigger. Made ground and reclaimed land around the harbours can settle unevenly, and a high water table affects some foundations.
Three checks the survey should cover:
- Tree species, distance from foundations, and any TPO (Tree Preservation Order)
- Drain condition and any documented leaks (CCTV survey if older clay drains)
- Crack pattern: BRE Digest 251 categories 0–5 with width, location, and direction
Mining-era subsidence in Portsmouth
No significant historic mining in or near Portsmouth. The Coal Authority CON29M is not required.
What subsidence means for your mortgage and insurance
Lenders treat historic, stabilised subsidence as standard if there is a structural engineer's report and any underpinning is documented. Active subsidence triggers retentions, specialist insurer placement, and in some cases lender refusal until remediation is complete and stable.
Insurance is the bigger ongoing constraint. A property with a prior subsidence claim sits in a constrained insurer market. The existing insurer typically continues cover but new business placement is harder. Disclosure of any prior claim is required on the seller's TA6 form.
How to check your specific address
City-wide context is orientation. Per-address checks before offer:
- 1Obtain address-specific ground susceptibility through a conveyancing environmental search or licensed BGS GeoSure shrink-swell data. GeoSure is a premium 1:50,000 product.
- 2Order a Coal Authority CON29M report if the property is in a historic coalfield boundary. Your conveyancer arranges this.
- 3Read the TA6 form for any prior subsidence claim, structural movement, or insurance involvement.
- 4Commission a RICS Level 3 (Building Survey) for any property over 60 years old in a high-clay-susceptibility area.
Run the check
Check subsidence signals for a UK address in 15 seconds
Directional BGS 1:1 million engineering geology, building age, tree context and the things to ask your surveyor.
Run a free previewFrequently asked questions
Is subsidence common in Portsmouth?
Portsmouth sits on Tertiary clays and sands of the Hampshire Basin (London Clay and Bracklesham/Bagshot beds), with made ground and reclaimed land in places around the harbours. These clays are shrink-swell susceptible, though the flat, sea-level setting makes flood the headline issue. BGS GeoSure rates clay shrink-swell susceptibility as moderate to high across parts of Portsmouth where Tertiary clays are near surface. Subsidence claim density is meaningful but secondary to coastal-flood exposure.
Will subsidence affect my mortgage in Portsmouth?
Lenders treat historic, stabilised subsidence as standard if a structural engineer's sign-off is in place. Active or progressive subsidence triggers retentions, specialist insurer placement, and in some cases lender refusal until remediation is complete.
What should the survey cover for subsidence in Portsmouth?
Trees on the Tertiary clays in proximity to foundations are the main shrink-swell trigger. Made ground and reclaimed land around the harbours can settle unevenly, and a high water table affects some foundations. The surveyor should record any cracks (BRE Digest 251 categories), assess proximity of trees and drains, and recommend a structural engineer's report where category 2+ cracking or active movement is suspected.
Compare nearby cities
Other local subsidence-risk guides to compare with Portsmouth
- Subsidence risk in Southampton, Hampshire
- Subsidence risk in Winchester, Hampshire
- Subsidence risk in London, Greater London
- Subsidence risk in Manchester, Greater Manchester
Keep going
Related Portsmouth buyer pages
- Flood risk in Portsmouth, the other major environmental check before offering.
- Full property check in Portsmouth , 12 public-record checks for any Portsmouth address.
- Subsidence monitoring on a survey , what surveyors mean by "monitor for movement" and what to do.
- Evidence of movement , the broader category surveyors use for cracking patterns.
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.
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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.
- Check this with: Environment Agency long-term flood risk mapOfficial flood-risk service for England, including river, sea, surface water, reservoir and groundwater where available.
- Data source: HM Land Registry Price Paid DataRegistered residential sale prices for England and Wales.
- Official register: Energy Performance Certificate RegisterPublic EPC certificate lookup for an address, postcode, street or certificate number.
- Data source: British Geological Survey Engineering Geology (1:1 million)National-scale bedrock and superficial-deposit mapping used as a directional ground indicator, not parcel-level GeoSure data or a site investigation.
- Data source: Copernicus CAMS via Open-MeteoCurrent modelled PM2.5, NO₂ and European AQI from a nearby CAMS grid cell.
- Data source: DEFRA Environmental Noise Directive mappingMapped road and rail noise contour data for England, classified into MyPropertyScan's low/moderate/high bands, and separate from the CAMS air-quality source.
- Data source: Police.uk crime dataOpen street-level crime data for England, Wales and Northern Ireland.
- Check this with: Ofcom broadband checkerOfficial checker for broadband availability and speeds.
- Check this with: Ofcom mobile coverage checkerOfficial predicted mobile coverage by network.
- Data source: Food Standards Agency food hygiene ratingsPublic register used to identify nearby food and drink venues.
- Official register: Ofsted inspection reportsSchool and provider inspection report lookup for England. Fee-paying schools come from the DfE Get Information About Schools register, since around half are inspected by ISI and carry no Ofsted grade.
- Official register: Historic England National Heritage ListListed buildings, scheduled monuments and other protected heritage entries in England.
General information only. Not legal, mortgage, insurance, or surveying advice. Always confirm with your own surveyor, broker, and conveyancer before making decisions. MyPropertyScan is operated by BiteRight Ltd.