Foundation cost for an extension is not really a rate, it is a depth question. Once the depth is known, the concrete volume, the muck away and the programme all follow from it, and the price becomes arithmetic.
We build extension foundations across Colchester, Chelmsford, Witham and the surrounding villages every month, and the pattern is consistent: the plots that surprise people are the ones with a mature tree, a shared drain or a neighbour's wall in the wrong place.
Typical extension foundation costs by depth
| Foundation depth | Typical cost per linear metre | When you see it |
|---|---|---|
| 900mm to 1200mm | £180 to £240 | Firm ground, no significant trees, straightforward single storey rear extension. |
| 1500mm to 1800mm | £250 to £340 | Shrinkable clay, common across much of north Essex. |
| 2000mm and beyond | £350 to £520 | Clay within influencing distance of mature trees, or made ground. |
| Piled foundations with ring beam | £12,000 to £26,000 for a typical extension | Where trees cannot be removed or ground is genuinely poor. |
Figures include excavation, concrete, reinforcement where specified and muck away. Building control inspections are included in our contracts as standard.
Why trees change everything
Shrinkable clay moves as it dries and rehydrates, and mature trees dry it far deeper than rainfall alone. Building control and your engineer will use the tree species, height and distance to set a depth, and an oak at nine metres will cost you considerably more than a birch at twelve.
Removing the tree is not automatically the answer either. Heave, where clay rehydrates and swells after a tree is felled, is a genuine risk and needs designing for.
Drains, and the surprise in most quotes
Extensions very often land on a foul run, and diverting or building over a public sewer needs agreement from the water authority. It is a manageable cost and a slow one to arrange, so it belongs in the programme from day one rather than as a discovery once the trench is open.
What our foundation package covers
- Setting out against boundaries and the existing structure
- Trench fill, strip or pad foundations to the engineer's design
- Reinforcement fixed to specification with correct cover
- Drainage diversions, chambers and building over agreements
- Oversite, insulation and floor structure
- Ten year written guarantee on the structural work we carry out
Where people try to save, and should not
Reducing depth, thinning reinforcement and skipping the engineer are all available savings and all of them are borrowing against year five. Foundations are the one element of a build that cannot be improved later without taking the building apart.
Services this relates to
Want the figure for your own property? Call 07770 880587 or email enquiries@breakinggroundltd.com and we will arrange a site visit.