When you’re building a home or extension, choosing the right foundation matters more than you might think. Get it wrong and you’ll face expensive repairs, structural problems, or both. Strip foundations and trench fill foundations are the two most common options for low-rise buildings in the UK, but they work differently, cost different amounts, and suit different situations.
This guide explains exactly what each foundation type is, when to use them, and which one makes sense for your project.
What Is a Strip Foundation?
A strip foundation is a continuous strip of concrete that runs underneath all load-bearing walls. Think of it as a concrete ribbon that supports the weight of your building.
Here’s how it works: you dig a trench (typically 450mm to 1000mm deep), pour a concrete base at the bottom (usually 150mm to 300mm thick), then build brick or block walls up from that concrete to ground level.
The concrete strip spreads the load from your walls across a wider area of soil. This prevents the building from sinking into the ground.
Key features:
- Narrow trenches (usually 600mm to 900mm wide)
- Relatively shallow excavation
- Brickwork or blockwork built from foundation up to ground level
- Less concrete used overall
Strip foundations have been the standard choice in British construction for decades. They work well in firm, stable ground where the soil can support the building’s weight without excessive settlement.
What Is a Trench Fill Foundation?
A trench fill foundation uses the same principle as a strip foundation but takes a completely different approach to construction.
Instead of building walls up from a concrete base, you dig a trench and fill almost the entire depth with concrete. The concrete typically stops about 150mm below ground level, where you then start building with bricks or blocks.
Key features:
- Same width as strip foundations (600mm to 900mm)
- Trenches filled almost completely with concrete
- Minimal or no brickwork below ground
- Much more concrete used
Trench fill became popular in the 1970s and 1980s when labour costs started rising faster than material costs. The idea is simple: use more concrete to save on labour time.
The Main Differences Between Strip and Trench Fill Foundations
| Feature | Strip Foundation | Trench Fill Foundation |
|---|---|---|
| Concrete volume | Low (150-300mm depth) | High (fills most of trench) |
| Below-ground brickwork | Significant (600-850mm typically) | Minimal (0-150mm) |
| Labour requirement | Higher (more bricklaying) | Lower (mostly concrete work) |
| Material cost | Lower | Higher |
| Construction speed | Slower | Faster |
| Ground conditions | Best in stable, dry soil | Better for wet or unstable soil |
| Depth flexibility | Limited | More adaptable |
The fundamental trade-off is simple: strip foundations use less material but need more skilled labour. Trench fill uses more material but gets the job done faster with less skilled work below ground.
When to Use Strip Foundations
Strip foundations make sense in specific situations:
Good ground conditions: If you’re building on firm clay, dense sand, or any stable soil with good bearing capacity, strip foundations work perfectly well. The ground can handle the load without problems.
Cost-conscious projects: When concrete prices are high relative to labour costs, strip foundations save money on materials. You’ll spend more on bricklaying, but the total cost often works out lower.
Shallow depths: For single-storey extensions or lightweight buildings where foundations don’t need to go deep, strip foundations provide adequate support without waste.
Dry sites: Strip foundations require dry working conditions. If the trenches stay dry during construction, bricklayers can work efficiently below ground.
Traditional construction: Some builders and structural engineers prefer strip foundations for traditional masonry buildings. The method has proven itself over many decades.
When to Use Trench Fill Foundations
Trench fill foundations solve specific problems:
Wet ground conditions: When trenches fill with water, trying to lay bricks below ground becomes a nightmare. Concrete doesn’t care about water during placement. This makes trench fill the obvious choice on sites with high water tables or in winter when trenches stay wet.
Unstable or variable soil: If ground conditions vary across your site or the soil is less predictable, filling the trench with concrete provides consistent support. You’re not relying on below-ground brickwork that might settle differently.
Faster construction schedules: Need to get out of the ground quickly? Trench fill gets you to ground level in days rather than weeks. This matters when you’re racing against weather or trying to meet tight deadlines.
Labour shortages: Finding skilled bricklayers who want to work in muddy trenches isn’t always easy. Trench fill reduces the need for below-ground masonry skills.
Deep foundations: When foundations need to go deeper than about 1 metre (perhaps due to nearby trees or poor surface soil), the cost difference between strip and trench fill narrows. At deeper levels, trench fill often becomes more economical because you’re saving significant labour time.
Cost Comparison: Which Foundation Costs Less?
The honest answer is: it depends on several factors that change constantly.
Material costs: Trench fill uses roughly 2 to 3 times more concrete than strip foundations. If concrete costs £100 per cubic metre and your foundation needs 20 cubic metres for trench fill versus 8 cubic metres for strip, that’s a £1,200 difference in concrete alone.
Labour costs: Strip foundations need skilled bricklayers working below ground for several days. Trench fill needs concreters for a day or two. If bricklayers cost £200 per day and you need them for 5 days versus 1 day, that’s an £800 saving with trench fill.
Ground conditions impact: Wet ground dramatically increases strip foundation costs. Bricklayers work slower in wet trenches, materials get damaged, and you might need pumping equipment. Trench fill costs stay relatively stable regardless of conditions.
Regional variations: In London and the Southeast, where labour costs run higher, trench fill often wins on price. In areas with lower labour costs, strip foundations frequently cost less.
Realistic example: For a typical 40 square metre single-storey extension with foundations 900mm deep, expect rough costs of £2,500-£3,500 for strip foundations versus £3,000-£4,000 for trench fill in average conditions. But in wet conditions, strip foundation costs can easily reach £4,500 while trench fill stays at £3,500.
The Building Research Establishment provides detailed guidance on foundation design that structural engineers use to calculate exact requirements for your specific site: https://www.bre.co.uk
Design and Structural Requirements
Both foundation types must meet Building Regulations Approved Document A requirements for structural safety.
Minimum dimensions: Foundations must be at least 150mm thick for strip foundations and wide enough to spread the load adequately. Width depends on ground bearing capacity and wall loads, typically ranging from 450mm for single-storey buildings to 900mm or more for two-storey buildings with heavy loads.
Depth requirements: Foundations must reach below the frost line (minimum 450mm in the UK) and sit on suitable load-bearing ground. Near trees, foundations need to go deeper to avoid problems from root activity and ground movement.
Concrete specification: Both types typically use C20/25 grade concrete minimum, though some engineers specify C25/30 for better durability. The concrete must resist sulfate attack if ground conditions warrant it.
Reinforcement: Neither standard strip nor trench fill foundations normally need reinforcement in typical domestic construction. However, stepped foundations, foundations on sloping sites, or situations with challenging ground conditions might require steel reinforcement bars.
Professional input: You’ll need a structural engineer or qualified designer to specify foundation dimensions based on soil investigation results and building loads. Building Control will want to inspect trenches before concrete gets poured.
Installation Process: Strip Foundations
Here’s what actually happens on site:
- Excavation: Diggers remove soil to create trenches of the specified width and depth. Trenches follow the line of all load-bearing walls.
- Inspection: Building Control inspects the trenches to verify they’ve reached suitable ground and meet the design specifications.
- Concrete pour: Ready-mix concrete fills the bottom of the trench to the specified depth (typically 150-300mm). The concrete is levelled and left to cure.
- Bricklaying begins: Once concrete has gained sufficient strength (usually after 24-48 hours), bricklayers build blockwork or brickwork up from the concrete to ground level.
- Damp proof course: A damp proof course gets installed at least 150mm above ground level.
- Backfill: Soil gets backfilled around the foundation once walls reach ground level.
The whole process typically takes 5-10 working days for a standard domestic extension, depending on size and weather.
Installation Process: Trench Fill Foundations
The trench fill process looks different:
- Excavation: Same as strip foundations – trenches dug to design depth and width.
- Inspection: Building Control inspects before any concrete goes in.
- Concrete pour: Ready-mix concrete fills the trench almost to the top, stopping about 150mm below ground level. This usually requires several loads of concrete for anything larger than a small extension.
- Brief wait: The concrete needs just enough time to set sufficiently for the next stage (often just 24 hours).
- Start building: Brickwork or blockwork starts from just below ground level.
- Damp proof course and backfill: Same as strip foundations.
Total time below ground gets cut to 2-4 working days typically. You’re building up from ground level much faster.
Ground Conditions and Soil Investigation
Neither foundation type works everywhere. The ground beneath your building determines what’s possible.
Clay soils: Both foundations work on clay, but clay shrinks when dry and swells when wet. This movement affects foundations, especially near trees. Strip foundations on shrinkable clay need careful design. Trench fill doesn’t solve clay movement problems – you might need deeper foundations or different solutions entirely.
Sandy soils: Dense sand provides excellent support for both foundation types. Loose sand might need wider foundations to spread loads effectively.
Rocky ground: Solid rock provides great bearing capacity but costs more to excavate. Strip foundations often make more sense because you’re not wasting expensive excavation by filling it with concrete.
Made ground: Fill material or disturbed ground needs investigation. You might need to excavate deeper to reach undisturbed natural soil. This often tips the balance toward trench fill.
Professional assessment: Get a proper soil investigation before finalising foundation design. A few trial pits or boreholes cost a few hundred pounds but prevent expensive mistakes. The Structural Engineers Association offers resources on foundation design standards that professionals follow: https://www.istructe.org
Common Problems and How to Avoid Them
Water ingress during construction: Strip foundations suffer badly if trenches flood. Have pumping equipment ready or schedule work for dry weather. With trench fill, you can concrete in wetter conditions, though standing water should still be pumped out before pouring.
Poor concrete placement: Concrete that isn’t properly compacted contains voids that weaken the foundation. Use poker vibrators when pouring, especially for trench fill where you’re placing large volumes.
Building too soon: Don’t build walls on fresh concrete. Wait at least 24 hours, longer in cold weather. Premature loading can damage the foundation.
Inadequate depth: Trenches that don’t reach suitable bearing ground cause settlement problems. If you hit soft spots, don’t ignore them. Dig deeper or get professional advice.
Poor drainage: Both foundation types need protection from water. Install proper drainage systems and ensure ground slopes away from buildings. Ground that stays saturated causes long-term settlement issues.
Tree proximity: Trees extract moisture from clay soil, causing it to shrink. Foundations within a distance equal to the mature tree height need special design. Ignoring nearby trees causes major problems later.
Alternative Foundation Types to Consider
Strip and trench fill aren’t your only options:
Raft foundations: A reinforced concrete slab that covers the entire building footprint. Used on poor ground or where loads must spread over a wide area. More expensive but sometimes necessary.
Pile foundations: Concrete columns driven deep into the ground to reach stable soil or rock. Essential for very poor ground conditions, heavy buildings, or challenging sites. Significantly more expensive than strip or trench fill.
Pad foundations: Individual concrete pads under point loads such as columns. Common in steel frame or timber frame construction. Not suitable for traditional masonry buildings.
Engineered solutions: Poor ground sometimes needs ground improvement, geogrid reinforcement, or other specialist techniques. A structural engineer will specify these when standard foundations won’t work.
For most domestic building projects on reasonable ground, strip or trench fill foundations provide adequate support at reasonable cost. The fancy alternatives only become necessary when ground conditions or building loads demand them.
Making Your Decision
Choose based on your specific situation:
Pick strip foundations if:
- Ground conditions are good and dry
- You want to minimise material costs
- Construction schedule allows time for below-ground brickwork
- Foundations are relatively shallow
- You have access to skilled bricklayers
Pick trench fill foundations if:
- Ground stays wet during construction
- You need faster construction
- Ground conditions are variable or uncertain
- Foundations need to go fairly deep (750mm or more)
- Labour costs are high relative to material costs
- You’re building in winter or in an exposed site
Get professional advice if:
- You’re unsure about ground conditions
- Trees are nearby
- The site slopes significantly
- You’re building near existing structures
- Ground appears to be made ground or previously disturbed
A structural engineer costs £300-£800 for foundation design on a typical domestic project. Building Control fees are unavoidable regardless. These costs are small compared to foundation failure repairs, which easily run into tens of thousands of pounds.
Summary: The Practical Choice
Strip foundations and trench fill foundations both do the same job through different methods. Strip foundations save on materials but need more labour and dry conditions. Trench fill foundations use more concrete but get you out of the ground faster and work in wetter conditions.
For most DIY builders and small contractors, trench fill foundations offer a safer, more practical choice. The extra concrete cost provides insurance against weather delays, wet ground, and labour problems. The faster construction schedule reduces risk and gets projects completed sooner.
However, in good, dry conditions with stable ground and available skilled labour, strip foundations remain a perfectly viable option that can save money.
Neither option is universally better. Assess your site conditions, check current material and labour costs, consider your construction schedule, and make an informed choice. When in doubt, consult a structural engineer who can design foundations specifically for your site and building.
The foundation is the one part of your building you absolutely cannot easily fix later. Spend the time and money to get it right first time.
Frequently Asked Questions
Can I use strip foundations for a two-storey extension?
Yes, strip foundations work fine for two-storey buildings if ground conditions are adequate. The foundations will need to be wider and possibly deeper than for single-storey construction to handle the increased load. A structural engineer will calculate the exact dimensions based on soil bearing capacity and building loads. Many two-storey homes across the UK sit on strip foundations built decades ago.
How deep do strip foundations need to go compared to trench fill?
Both types typically go to the same depth because depth depends on ground conditions, frost protection, and proximity to trees, not on the foundation type itself. The difference is what you do with the trench once it’s dug. Minimum depth in the UK is 450mm, but 750mm to 1000mm is more common for typical house foundations. Near trees, depths of 2 metres or more might be necessary.
Does trench fill crack more than strip foundations?
No, properly designed trench fill foundations don’t crack more than strip foundations. Both can develop cracks if the concrete mix is wrong, if there’s inadequate curing, or if ground movement occurs. The larger volume of concrete in trench fill might actually provide slightly better crack resistance due to the mass of concrete. Use the correct concrete grade and ensure proper placement regardless of foundation type.
Can I change from strip to trench fill during construction?
Yes, this happens regularly when ground conditions turn out worse than expected. If trenches start flooding or you discover soft ground, switching to trench fill makes sense. You’ll need to inform Building Control of the change and adjust your concrete order. The main issue is cost – you’ll suddenly need much more concrete than planned, so make sure you have budget flexibility.
Do I need reinforcement in either foundation type?
Standard strip and trench fill foundations for typical domestic buildings don’t normally need reinforcement. However, stepped foundations on slopes, foundations bridging over soft spots, or foundations designed to resist specific ground movements might need steel reinforcement bars. Your structural engineer will specify reinforcement if the design requires it. Don’t add or omit reinforcement without professional guidance.