Subsidence Explained: Everything You Need to Know

Subsidence is the word that strikes fear into every property buyer's heart. It's one of the most serious structural issues a property can face, potentially costing tens of thousands to repair and making properties unmortgageable. Understanding subsidence could save you from the most expensive mistake of your life.

What Is Subsidence?

Subsidence is the downward movement of the ground beneath a building, causing the foundations to sink and the structure above to crack and deform. Unlike settlement (which is gradual and stops), subsidence is active, progressive, and can cause severe structural damage.

Types of Ground Movement

• Subsidence: Ground sinking beneath foundations (most serious)
• Settlement: Natural gradual sinking after construction (usually stabilizes)
• Heave: Ground expanding upward (opposite of subsidence)

Common Causes of Subsidence

1. Clay Soil Shrinkage (Most Common)

Clay soils expand when wet and shrink when dry. Extended dry periods cause clay to contract, removing support from foundations.

• Risk areas: London clay, Oxford clay, Lias clay (common in Bedfordshire, Buckinghamshire)
• Seasonal variation: Worse in hot, dry summers
• Tree impact: Tree roots extract moisture, accelerating shrinkage

2. Tree Root Damage

Large trees near properties on clay soil are a major cause of subsidence:

• High-risk trees: Oak, willow, poplar, ash, elm
• Risk distance: Trees within 1.5x their mature height of the property
• Example: A 15m tall oak tree poses risk to properties within 22.5m

3. Leaking Drains

Water from leaking drains washes away soil particles beneath foundations (erosion subsidence).

4. Mining Activity

Old mine workings can collapse, causing surface subsidence decades later.

5. Poor Construction

Inadequate foundations for soil type or building weight.

Warning Signs of Subsidence

External Signs

• 📏 Diagonal cracks wider than 5mm, especially near corners
• 📏 Stepped cracking in brickwork following mortar lines
• 📏 Cracks wider at top than bottom (classic subsidence pattern)
• 📏 Leaning or bulging walls
• 📏 Cracks in paving/paths near the building

Internal Signs

• 🚪 Doors and windows sticking or not closing properly
• 🚪 Doors jamming in frames
• 🚪 Cracks in walls, especially around doors and windows
• 🚪 Gaps appearing between walls and ceilings/floors
• 🚪 Sloping floors
• 🚪 Cracking in internal plasterwork

Subsidence vs Historic Settlement

What To Do If You Suspect Subsidence

Step 1: Get a Professional Survey

Commission a RICS Level 3 Building Survey from a qualified surveyor experienced in structural issues.

Step 2: Crack Monitoring

Surveyor may recommend monitoring over 6-12 months:

• Install crack monitoring strips across affected areas
• Document crack width and changes over time
• Note seasonal variations

Step 3: Structural Engineer's Report

For confirmed subsidence, a structural engineer will:

• Carry out detailed investigation
• Possibly dig trial pits to inspect foundations
• Identify cause (trees, drainage, soil, etc.)
• Recommend remedial solution

Subsidence Repair Methods

1. Underpinning (Most Common)

What it is: Extending foundations deeper into stable ground

• Process: Excavate beneath existing foundations in sections, pour new concrete base
• Duration: 4-12 weeks depending on extent
• Cost: £10,000 - £50,000+ depending on property size
• Effectiveness: Very high - permanent solution

2. Tree Management

If trees are the cause:

• Remove problematic trees (with care - can cause heave)
• Prune to reduce water demand
• Install root barriers
• Cost: £500 - £3,000

3. Drainage Repairs

If leaking drains are the cause:

• Repair or replace damaged drainage
• Monitor for stabilization (6-12 months)
• Cost: £1,000 - £5,000

4. Resin Injection

Modern alternative to underpinning:

• Inject expanding resin beneath foundations
• Stabilizes ground and lifts foundation
• Faster than underpinning (days vs weeks)
• Cost: £5,000 - £15,000

Impact on Property Value and Insurance

Mortgage Implications

• ❌ Properties with active subsidence are usually unmortgageable
• ✅ Properly repaired subsidence with guarantees can be mortgaged
• ⚠️ Lenders may require structural engineer's confirmation of stability

Insurance Considerations

• Properties with subsidence history face higher premiums
• May need specialist insurance (£800-£2,000+ annually vs £200-£400 standard)
• Subsidence cover may have high excess (£1,000-£5,000)
• Some insurers refuse coverage for properties with subsidence history

Property Value Impact

• Active subsidence: 20-50% reduction in value (often unsaleable)
• Repaired subsidence: 5-20% reduction
• Well-documented repair with guarantee: 0-10% reduction

Buying a Property with Subsidence

Red Flags - Walk Away If:

• ❌ Active subsidence with no identified cause
• ❌ Seller cannot provide repair evidence or guarantees
• ❌ Multiple structural issues beyond subsidence
• ❌ Property unmortgageable

Proceed with Caution If:

• ⚠️ Subsidence properly repaired with 10+ year guarantee
• ⚠️ Cause identified and remedied (tree removed, drains fixed)
• ⚠️ Engineer confirms stability
• ⚠️ Significant price reduction to reflect history

Questions to Ask Seller:

1. When was subsidence discovered?
2. What was the cause?
3. What repairs were carried out and by whom?
4. Do you have guarantees/warranties for the work?
5. Has monitoring been carried out post-repair?
6. What is the current insurance situation?
7. Were Building Control notified and involved?

Prevention: Protecting Your Property

• ✅ Maintain adequate distance between trees and property
• ✅ Ensure good drainage around foundations
• ✅ Fix leaking gutters/downpipes promptly
• ✅ Don't allow soil levels to drop near foundations
• ✅ Monitor for cracks and address early
• ✅ Keep drains well-maintained