How to Price a Strip Foundation: Labour, Materials and Margin Guide

Summary

Strip foundations are the default foundation type for domestic extensions and small new-build work, and pricing them well comes down to getting three variables right: trench width and depth (read from Table A1, not guessed), spoil removal (frequently underestimated, especially on deep clay trenches), and access for concrete delivery (pump, barrow run, or wheeled skip — each with a different cost and time profile).

Because Approved Document A provides prescriptive widths for standard ground conditions, strip foundations are cheaper to design than a raft (no structural engineer's fee in straightforward cases) but the pricing risk shifts to the ground itself — if what's excavated doesn't match the assumed soil classification, depth and width both change, and by then the trench is already open. This guide covers pricing excavation and concrete by depth/width band, what drives spoil disposal cost, concrete delivery options, and what happens when ground conditions don't match the table.

Key Facts

• Approved Document A, Table A1 — gives prescriptive strip foundation widths for domestic masonry construction based on soil type and number of storeys; see strip foundation design for the full width table
• Minimum width — 300mm minimum in virtually all cases, increasing with poorer ground and higher wall loads (up to 750mm for soft clay/soft sand under a 3-storey wall)
• Minimum depth (non-clay, frost protection) — 450mm below ground level; up to 600mm in exposed/cold locations
• Minimum depth (shrinkable clay) — 750-900mm typically, rising to 1,000mm+ near mature trees on high-shrinkage clay — see strip foundation design for the tree-proximity depth table
• Concrete grade — typically ST2/GEN3 for standard domestic strip foundations; higher sulphate-resisting mixes required on made ground or brownfield sites (DC-2/DC-3 classifications)
• Concrete volume scales fast with depth — a trench at 900mm deep uses double the concrete of the same trench at 450mm, so clay-soil jobs cost meaningfully more even at the same wall length
• Muck-away/spoil removal — priced by volume (m³) and distance to registered tip; a deep clay trench on a small site with no room to store spoil can make disposal the largest single cost on the job
• Concrete delivery method — ready-mix pumped directly into trench (fastest, needs pump access and hardstanding for the wagon), wheeled/barrowed from a road-side pour (needs more labour, suits restricted access), or site-mixed (rare on domestic jobs now, generally more expensive per m³ once labour is counted)
• Trench width vs foundation width — the excavated trench is typically wider than the concrete foundation width to give operatives room to work; don't price excavation volume using the Table A1 concrete width alone
• Building control inspection — mandatory foundation-stage inspection of the excavated, prepared trench before concrete can be poured; factor inspection lead time into programme
• Trench safety — trenches deeper than 1.2m require side support or safe battering unless the ground is self-supporting; price temporary support where depth or ground condition requires it
• Reinforcement — not normally required for standard domestic strip foundations designed to Table A1 (mass concrete); only added where the structural engineer specifies it for variable ground or unusual loads
• VAT — standard-rated at 20% for extensions and most domestic groundworks
• Margin — 15-20% on straightforward Table A1 work; step up where ground conditions are uncertain or access for muck-away/concrete delivery is restricted

Quick Reference Table

Figures assume normal working access for a mini-excavator and concrete pump/wagon. A typical single-storey rear extension needs roughly 12-20 linear metres of strip foundation depending on footprint and return walls.

Detailed Guidance

Reading Table A1 Correctly Before Pricing

The width (and by extension the concrete volume) comes directly from Approved Document A Table A1, cross-referencing soil classification against the number of storeys the wall supports. This only applies where the site meets Table A1's conditions: traditional masonry construction, uniform ground with no made ground or unusual loads, and a classification the excavated soil clearly matches (rock, gravel/sand, stiff clay, soft clay, or very soft — see strip foundation design for the classification test). If the ground doesn't clearly match one of these categories once excavated, stop pricing from the table and get a structural engineer involved — forcing a Table A1 price onto ground that doesn't qualify is a common cause of building control rejecting the foundation at inspection.

Depth Drives Cost More Than Width

Because concrete volume increases with both width and depth, but depth requirements vary far more dramatically by soil type (450mm vs 1,000mm+ on high-shrinkage clay near mature trees) than width does, depth is the variable that most changes the price of an otherwise similar-looking job. Always establish tree proximity and soil type at the quoting stage — a soil survey or trial pit, even an informal one, before pricing a job on clay-heavy ground avoids discovering a 900mm requirement has become 1,200mm+ once the trench is open and the engineer or building control officer gets involved.

Spoil Removal — Price the Actual Volume, Not a Guess

Trench excavation on a strip foundation produces a predictable volume of spoil that scales with width, depth, and total run length — calculate it rather than estimating "a few loads." Where the site has room to store or reuse spoil (levelling, garden landscaping) this can meaningfully reduce disposal cost; where there's no storage space and everything has to leave site, muck-away cost (skip, grab lorry, or wheeled barrow to a road-side collection point) should be priced by m³ against current local tip/landfill rates, which rise annually.

Concrete Delivery — Match the Method to Access

A site with clear vehicle access for a concrete wagon and pump line is the cheapest and fastest way to fill a strip trench — direct pumping avoids double-handling. Restricted access (narrow side passage, no direct trench access from the road) forces either a longer pump line/boom (extra hire cost) or barrow-run labour, which is slower and more labour-intensive per m³ placed. Assess access at quote stage, not on pour day — discovering the wagon can't get within pump range on the morning of the pour is an expensive and entirely avoidable delay.

What to Include in the Quote

A complete strip foundation quote should show: excavation and disposal as a discrete line (with the assumed soil classification and depth stated), concrete supply and placement method, any temporary trench support required by depth/ground condition, building control inspection allowance, and a clearly stated assumption about ground conditions with a provisional sum or day-rate contingency if the ground hasn't been formally investigated — this is the standard way to protect margin on a job type where the biggest cost risk (what's actually in the ground) isn't visible until the trench is dug.

Frequently Asked Questions

Can I price a strip foundation without a soil survey?

For most straightforward domestic extensions, yes — an experienced groundworker can classify the soil on site once excavation starts and confirm it against Table A1. The risk is pricing before excavation starts based on an assumption; where there's any doubt (known clay area, nearby trees, previous ground movement on the property or neighbours), get a trial pit dug or a soil assessment before fixing a firm price.

What happens if the trench reveals worse ground than expected?

Stop and notify building control before pouring. Options are to excavate deeper to reach suitable bearing ground, widen or reinforce the strip, or move to a different foundation type (commonly a raft — see raft foundation pricing guide) if the ground genuinely won't support a strip foundation design. Any of these changes the price, and this should be flagged as a provisional risk in the original quote rather than absorbed.

Do strip foundations need reinforcement?

Not normally, when designed within Table A1 limits — they're poured as mass (unreinforced) concrete. Reinforcement is added only where a structural engineer specifies it, typically for variable ground conditions, unusually heavy loads, or where the foundation is close to Table A1's limits and the engineer wants additional safety margin.

How much does foundation depth near a tree add to the price?

Significantly. A standard shrinkable-clay depth of 750-900mm can rise to 1,000-1,500mm or require specialist engineering within roughly one to two times the mature height of a nearby tree, per NHBC guidance. This roughly doubles or more the concrete volume (and cost) for that section of trench compared with standard clay depth — always check tree proximity, including trees on neighbouring land, before quoting.

Regulations & Standards

• Building Regulations 2010 Approved Document A — Structure; Table A1 prescriptive strip foundation widths and depth guidance

• BS 8004:2015 — Code of practice for foundations; detailed technical standard behind Table A1

• BS 8500-1:2015+A2:2019 — Concrete specification, including sulphate-resisting mixes for made ground/brownfield sites

• NHBC Standards Chapter 4.2 — Building near or over trees; foundation depths on shrinkable clay soils

• CDM Regulations 2015 — trench safety and excavation planning duties

• Approved Document A - GOV.UK — full Table A1 and depth guidance

• NHBC Standards - Chapter 4.2 — foundations near trees on shrinkable clay

• gov.uk - Landfill Tax rates — current spoil disposal cost driver

• strip foundation design — full Table A1 width table, depth guidance, and soil classification method

• raft foundation pricing guide — cost comparison for the alternative foundation type on poor ground

• foundation depths and types — overview of foundation types and when each applies

• demolition pricing guide — related groundworks pricing where a strip foundation follows a demolition/strip-out phase