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Subsidence and Structural Movement: Detection and Solutions

January 2025 14 min read
Subsidence detection and repair
1 in 50
UK Homes Affected
£15k-£50k+
Average Repair Cost
50-70%
Tree-Related
6-12 months
Monitoring Period

Subsidence is one of the most serious and costly defects that can affect a property. Understanding the signs, causes, and solutions is essential for property buyers and owners in Milton Keynes and surrounding areas.

Important: Subsidence is downward ground movement beneath a building's foundations. It differs from settlement (normal initial compression) and heave (upward ground movement). Early detection is crucial to prevent extensive damage.

Types of Structural Movement

Not all structural movement is subsidence. Understanding the different types helps determine the appropriate response:

Subsidence - Downward Movement

Definition: Downward movement of the ground beneath a building's foundations causing the building to sink.

Main Causes:

  • Clay soil shrinkage (especially during dry summers)
  • Tree roots extracting moisture from soil
  • Leaking drains washing away soil
  • Mining or quarrying activities (historical)
  • Soil compaction or erosion

Typical Signs: Cracks wider at top than bottom, diagonal cracks near doors/windows, doors and windows sticking.

Severity: HIGH - Can cause significant structural damage requiring expensive repairs (£15,000-£50,000+).

Heave - Upward Movement

Definition: Upward movement of the ground, typically when clay soils expand after moisture returns.

Main Causes:

  • Removal of trees (soil moisture increases)
  • Repair of leaking drains (moisture returns)
  • Extended wet periods after drought
  • Clay soil expansion

Typical Signs: Cracks wider at bottom than top, floors lifting, doors and windows jamming, rippling in floors.

Severity: MODERATE-HIGH - Less common than subsidence but can still cause significant damage.

Settlement - Normal Compression

Definition: Normal downward movement as new buildings adjust to their foundations, or minor ongoing compression.

Main Causes:

  • Weight of new building compressing soil
  • Normal ground consolidation
  • Minor soil compression over time

Typical Signs: Hairline cracks, usually uniform across the building, stable (not progressive).

Severity: LOW - Usually not a concern. Most buildings experience minor settlement in first few years.

Key Difference: Settlement is expected and stable. Subsidence is progressive and requires intervention.

Common Causes of Subsidence in Milton Keynes

Milton Keynes properties, particularly those built on clay soils, face specific subsidence risks:

1. Clay Soil Shrinkage (Most Common)

Much of Milton Keynes is built on Oxford Clay, which is highly reactive to moisture changes:

  • Dry periods: Clay shrinks as moisture is extracted, creating voids beneath foundations
  • Tree roots: Trees extract moisture up to 1.5x their mature height distance from the trunk
  • Seasonal variation: Particularly problematic during hot, dry summers
  • Affected areas: Properties within 30 meters of large trees on clay soil
High-Risk Trees: Oak, willow, poplar, elm, and ash are particularly problematic. Mature trees can affect properties 20-30 meters away.

2. Leaking Drains and Water Mains

Water escaping from damaged drainage systems can wash away supporting soil:

  • Drain defects: Cracked or collapsed drains allow water to escape
  • Soil erosion: Water washes away fine particles, creating voids
  • Secondary effects: Can also cause heave when repaired (moisture returns)
  • Detection: High water bills, damp patches, sewage odors, or unexplained lush vegetation

Solution: CCTV drain survey (£200-£400) can identify leaks. Repairs typically £1,000-£5,000 depending on extent.

3. Historical Mining and Quarrying

Some areas of Buckinghamshire have historical mining activities:

  • Historical extraction: Clay pits, chalk quarries, and brick-making sites
  • Ground voids: Old workings can collapse creating subsidence
  • Research essential: Coal Authority and British Geological Survey searches
  • Insurance: May be more difficult to obtain in known mining areas

Action: Always conduct mining searches (part of standard conveyancing) and ask surveyors about local geological risks.

How to Detect Subsidence: Warning Signs

Early detection is crucial. Look for these warning signs:

Sign What to Look For Severity Indicator
Crack Width • >3mm: Minor
• 5-15mm: Moderate
• 15-25mm: Severe
• >25mm: Very severe 		Wider cracks = more serious
Crack Pattern Diagonal cracks, often wider at top, near corners of windows/doors Diagonal = possible subsidence
Vertical = likely thermal/settlement
Crack Location External walls, extensions, where new meets old construction External + internal = more serious
Doors & Windows Sticking, jamming, gaps appearing above frames New sticking = active movement
Floor Problems Sloping floors, cracks in tiles, gaps between floor and skirting Multiple floors = widespread issue
Exterior Signs Leaning or bulging walls, cracks in brickwork, rippling wallpaper Bulging = urgent structural concern
Critical Warning Signs Requiring IMMEDIATE Professional Assessment:
  • Cracks wider than 15mm (about the width of a £1 coin)
  • Cracks that are growing rapidly (visible changes over weeks)
  • Bulging or leaning walls
  • Multiple areas of cracking throughout the property
  • Doors and windows that suddenly won't close

Interactive Subsidence Risk Assessment

Assess Your Property's Subsidence Risk

Answer these questions to understand your property's subsidence risk level:

Tree Distance Safety Calculator

Calculate Safe Tree Distance

Trees are the leading cause of subsidence on clay soils. Use this calculator to determine if trees pose a risk:

The Investigation Process

When subsidence is suspected, a systematic investigation is essential:

1

Initial Survey & Assessment

What happens: RICS chartered surveyor inspects property, measures cracks, assesses severity

Cost: £500-£800

Duration: 2-3 hours on site, 1 week for report

Outcome: Preliminary opinion on cause and severity

2

Crack Monitoring

What happens: "Tell-tales" or glass strips placed across cracks to monitor movement

Cost: £200-£400 for installation

Duration: Typically 6-12 months (must cover seasonal variations)

Outcome: Determines if movement is active or historic

3

Site Investigation

What happens: Trial pits or boreholes to examine soil conditions and foundations

Cost: £1,500-£3,000

Duration: 1-2 days on site, 2 weeks for laboratory analysis

Outcome: Detailed soil analysis and foundation assessment

4

Root Investigation (if trees implicated)

What happens: Excavation to identify tree root presence near foundations

Cost: £800-£1,500

Duration: 1 day

Outcome: Confirms if trees are causing the problem

5

Structural Engineer Report

What happens: Analysis of all investigation data, recommended solutions

Cost: £1,000-£2,000

Duration: 2-3 weeks

Outcome: Detailed specification for repairs with cost estimates

Total Investigation Timeline: From initial survey to final engineer's report typically takes 9-15 months due to the essential monitoring period. Fast-track is possible in severe cases but not recommended.

Repair Solutions and Costs

Treatment depends on the cause and severity. Main options include:

Solution When Used Process Typical Cost
Monitoring Only Historic movement, now stable Continue monitoring, address cause (trim trees, repair drains), no structural work £500-£2,000
Crack Repair & Redecoration Minor subsidence, now stable Fill cracks, replaster, redecorate. May include masonry stitching £2,000-£8,000
Mass Concrete Underpinning Moderate to severe subsidence Excavate beneath foundations in sections, fill with concrete to deeper, stable soil £15,000-£30,000
Mini-Piled Underpinning Severe subsidence, difficult access Install steel piles into ground, connect to foundation with needle beams £25,000-£50,000+
Resin Injection Void filling, soil stabilization Inject expanding resin to fill voids and stabilize ground £8,000-£15,000
Tree Management Tree-related subsidence Pruning, pollarding, or removal. Note: removal can cause heave £500-£3,000 per tree

Underpinning: What You Need to Know

Underpinning is the most common major repair for subsidence. Here's what it involves:

  • Process: Foundations are extended downward to reach stable soil/rock
  • Duration: Typically 6-12 weeks depending on extent
  • Disruption: Significant - excavation around property, noise, dust, scaffolding
  • Occupation: May be able to remain in property, but often better to move out
  • Guarantee: Specialist contractors typically provide 10-year guarantee
  • Insurance: Usually covered by buildings insurance (after excess)
Impact on Property Value: Even after successful underpinning, subsidence history can affect saleability and value (typically 10-25% reduction). Future insurance may be more expensive. Full disclosure is legally required.

Making an Insurance Claim

Most buildings insurance policies cover subsidence. Here's how to navigate the claims process:

Step 1: Notify Your Insurer Immediately

What to do:

  • Contact insurer as soon as subsidence is suspected
  • Take photographs of all cracks and damage
  • Document dates when cracks were first noticed
  • Don't arrange repairs before insurer inspects

What happens: Insurer will appoint a loss adjuster and structural engineer to assess the claim.

Step 2: Investigation and Monitoring

What to expect:

  • Insurer's surveyor will inspect the property
  • Crack monitoring will be installed (6-12 months typically)
  • Site investigations may be required
  • You may be asked to address causes (e.g., tree pruning, drain repairs)

Your rights: You can instruct your own surveyor at your expense for an independent opinion. This can be valuable if you disagree with the insurer's assessment.

Step 3: Claim Settlement and Repairs

If claim accepted:

  • Cash settlement: Insurer pays repair costs minus excess (typically £1,000)
  • Managed repairs: Insurer appoints contractors and manages the work
  • Betterment: You may have to pay for improvements beyond original specification
  • Alternative accommodation: Often covered if you need to move out during repairs

Typical excess: £1,000 for subsidence claims (check your policy).

Important: Future insurance will be more expensive and you must declare subsidence history when selling. Get written confirmation when repairs are complete.

Common Claim Rejection Reasons

Insurers may reject claims if:

  • Wear and tear: Damage is due to age, weathering, or poor maintenance
  • Settlement not subsidence: Normal settlement is not covered
  • Pre-existing: Damage existed before policy started (non-disclosure)
  • Poor design/construction: Inherent defects not covered
  • Deliberate acts: You caused the damage (e.g., failed to maintain drains)

If claim refused: Request detailed written reasons, get independent surveyor opinion, consider Financial Ombudsman if dispute cannot be resolved.

Preventing Subsidence

While you can't control all factors, these measures reduce subsidence risk:

Subsidence Prevention Checklist

Buying a Property with Subsidence History

Properties with subsidence history are harder to sell and usually sell for less, but may still be acceptable investments if properly repaired:

Essential Checks Before Proceeding

  • Get structural engineer report: Independent assessment of repairs (£800-£1,200)
  • Review original investigation reports: Understand the cause and whether it's resolved
  • Check repair guarantees: Minimum 10 years remaining, transferable to you
  • Verify monitoring: Has property been stable since repairs? Get evidence
  • Insurance availability: Contact insurers to confirm you can get cover and costs
  • Cause still present?: Are trees still there? Have drains been repaired?

Price Negotiation Strategy

  • Typical reduction: 10-25% below market value depending on severity and history
  • Recent repairs: Less reduction if repairs completed recently with full guarantees
  • Old history: If repairs were 20+ years ago and stable since, smaller reduction justified
  • Active subsidence: Substantial reduction (30-50%) or walk away if not addressed
  • Get quotes: If repairs needed, get contractor quotes to evidence negotiation
  • Factor insurance: Higher ongoing costs should reduce price

Negotiation leverage: Limited buyer pool (most will reject), mortgage difficulties, insurance issues all justify substantial discount.

Mortgage and Insurance Challenges

Mortgages:

  • Many lenders refuse properties with subsidence history
  • Those that will lend typically require larger deposits (25-40%)
  • Higher interest rates may apply
  • Specialist lenders more likely to consider
  • Valuer must confirm repairs satisfactory

Insurance:

  • Higher premiums (50-200% more than equivalent property)
  • Higher excess for subsidence claims (£2,000-£5,000)
  • Some insurers exclude subsidence cover entirely
  • May need specialist insurers
  • Must disclose history to all future buyers

When Buying: Seller Obligations

Legal Requirements: Sellers MUST disclose subsidence history in the TA6 Property Information Form. Failure to disclose is misrepresentation and can lead to legal action. This includes historic issues even if repaired.
Milton Keynes Surveyors Subsidence Service:

Our RICS chartered surveyors have extensive experience with subsidence in Buckinghamshire properties:

  • Preliminary subsidence assessments (£500-£800)
  • Full structural surveys including subsidence risk analysis
  • Crack monitoring and reporting services
  • Independent assessment of previous subsidence repairs
  • Expert witness services for insurance disputes
  • Pre-purchase advice on properties with subsidence history

Don't take risks with subsidence. Contact us for expert assessment.

Frequently Asked Questions

How quickly does subsidence develop?

Subsidence typically develops gradually over months or years, not overnight. Tree-related subsidence often occurs during hot, dry summers when clay soils shrink most. You may notice cracks appearing or widening over a season. Rapid cracking (visible changes over days/weeks) suggests active movement requiring immediate professional assessment.

Can subsidence be completely fixed?

Yes, subsidence can be successfully repaired, but the property will always have a subsidence history. Underpinning addresses the structural issue, and if the cause (e.g., trees, drains) is also resolved, the property should be stable. However, complete "fixing" means the risk is minimized, not eliminated. Proper monitoring and addressing the underlying cause are essential.

Should I walk away from a property with subsidence?

It depends on the severity and repair status:

  • Walk away: Active subsidence not being addressed, severe damage, cause not resolved, no guarantees
  • Negotiate hard: Historic subsidence with proper repairs and guarantees (expect 10-25% discount)
  • Proceed with caution: Minor historic issues, fully repaired 10+ years ago, stable since

Always get independent structural engineer advice. Factor in difficulty selling in future and higher insurance costs.

How much does subsidence devalue a property?

Typical devaluation ranges from 10-25% depending on factors:

  • 10-15% reduction: Historic, properly repaired, good guarantees, stable for 5+ years
  • 15-20% reduction: More recent repairs (2-5 years ago), shorter guarantee period
  • 20-30% reduction: Recent repairs, complex history, cause not fully resolved
  • 30-50% reduction: Active/ongoing subsidence requiring further work

Properties with subsidence history also take longer to sell (3-6 months longer on average) due to limited buyer pool and mortgage difficulties.

Concerned About Subsidence?

Our RICS chartered surveyors provide expert subsidence assessments throughout Milton Keynes, Bedford, Northampton, and Buckinghamshire.

Book Your Subsidence Assessment