What this guide covers: extension foundations cost · UK planning context · what to ask in a quotation
Quick budgeting answer: Extension costs must be compared on a like-for-like scope. A finished room, weather-tight shell and supply-only kit are different products; the model gives broad scenarios, not a contractor quote.
Broad groundworks share in our model, not a foundations-only quotation
What counts as extension foundation work?
The package can include site clearance, setting out, excavation, spoil removal, trench supports, concrete, reinforcement, blockwork below ground, damp-proofing and the ground floor structure. Drain diversions and service trenches may be priced separately. Check the contractor’s schedule before comparing a “foundations only” number with a full groundwork quote.
Why price per metre is misleading
Two extensions with the same footprint can need different foundation depth or type. Nearby trees, made-up ground, clay, changes in level, adjacent structures and sewers all matter. A structural engineer and building control may require a deeper trench or an engineered alternative. This is why an estimate without site information must remain broad.
In our model, site setup, groundworks and foundations together account for an illustrative 18% share of base construction costs. That is an allocation for explanation—not data showing that foundations always cost 18%, nor a substitute for a measured groundwork quotation.
Tree species, distances and shrinkable soil may influence the foundation specification.
Shared sewers, inspection chambers and old pipes may require investigations or permissions.
Can a mini excavator, concrete lorry or pump reach the site without extra handling?
New excavations and extensions near the current house require suitable temporary and permanent support.
Check the evidence before fixing a budget
Start with drainage records and a measured plan. An engineer or surveyor may recommend trial pits or other investigation. Agree who communicates with building control about inspections before concrete is poured. Keep a contingency for unexpected ground conditions, and do not ask a builder to guarantee unseen ground on a guessed footing depth.
| Ask about | Why it matters |
|---|---|
| Scope of excavation | Is trench depth, spoil disposal and any temporary shoring defined? |
| Concrete and reinforcement | Are supply, placing, pump hire and inspection access included? |
| Drain and utility work | Are surveys, diversions, protection and connections provisional or fixed? |
| Unforeseen conditions | What unit rates or variation process applies if the engineer changes the design? |
How foundations feed into a total extension budget
Foundations are one item in the complete single-storey extension or two-storey extension budget. Confirm that the same groundwork allowance is not charged once within the shell rate and again as an extra. For an over-garage build, existing foundation capacity needs separate investigation.
Calculate your own extension budget
Our published model provides broad planning scenarios; it does not replace a measured survey, specialist design or itemised builder quotations.
Calculate my extension price →Common questions
How much do foundations cost for a 20m² extension?
There is no reliable fixed total from floor area alone. Ground conditions, tree influence, design and access need to be assessed; obtain an itemised groundwork price.
Does building control inspect extension foundations?
Inspection arrangements depend on the approval route and jurisdiction; clarify required stages with the appointed building-control body before covering work.
Are foundations included in the extension calculator?
Yes, in its general base build. Its 18% breakdown label groups setup, groundworks and foundations as an illustrative allocation, not a standalone market price.
Pricing context: Federation of Master Builders; full-project planning scenarios and their assumptions are described in our pricing methodology. For permissions in England, check the Planning Portal and GOV.UK building-control guidance. Rules differ across the UK. All figures and design comments here are general context, not measured quotes or professional advice.