Can I argue against piling to save cost?

Rarely successfully — building control and warranty providers (NHBC, LABC) follow established guidance. Removing the tree (with TPO/CA checks) is the only legitimate route to strip foundations.

How much do I save with strip foundations?

Typically £6,000–£18,000 on a single-storey extension. But uninsurable subsidence later costs vastly more.

Does Builderr advise on foundation choice?

Yes — included in design stage with soil investigation and engineer's foundation strategy.

What if my house is on piles already?

Common in basement-converted London terraces. New extension piles tie into existing scheme via engineer's design.

Quick Answer

Mini-pile vs traditional foundation — which is right for a London extension?

Mini-piles cost more than traditional strip foundations (typically 30–80% premium) but solve London's tree-clay constraints. Traditional strip foundations: £180–£260 per linear metre, 1.0–1.8m depth, no trees within influencing distance, stable clay or gravel. Mini-piles: £180–£280 per linear metre + ground beams, 6–12m depth, mandatory near mature trees or on weak strata. London Clay + trees usually forces piles.

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Strip foundations

Trench dug 1.0–1.8m deep, concrete poured. Cheapest option (£180–£260/m). Works on stable ground with no nearby trees. Most outer-London 1930s semis sit on strip foundations. Deepening beyond 2.5m gets uneconomic — at that point piles win.

Mini-piles

Drilled to bearing strata 6–15m down, capped with ground beams and slab. Premium of 30–80% over strip but de-risk tree-clay interaction. Mandatory under NHBC and Eurocode where trees within influencing distance (mature oak: 20m+; sycamore: 12m+; smaller species 6–10m).

Decision drivers

Soil investigation (mandatory) reveals strata, water table and shrinkability. Tree survey identifies species, age and distance. Engineer applies NHBC tree guide to map zone of influence. Strip viable if soil OK + no trees in zone; otherwise piles.

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Senior consultant call within one business hour. Free desk-based planning assessment. Fixed-scope quote — no provisional sums, no day-rate creep.