Foundation Cost Calculator

Extension foundations are one of the hardest costs to pin down before the trench is open, because nobody knows exactly what the ground is doing until the digger starts. For a standard single-storey rear extension, the biggest line is the concrete itself at £105 – £140 per m³, poured into a trench-fill foundation roughly 600mm wide and at least 1m deep on stable ground. On top of that sits groundworker labour at £200 – £280 per day and mini-digger hire at £125 – £250 per day. This calculator takes your trench length, ground condition, and site access and turns those rates into a realistic ballpark.

How the foundation cost breaks down

The concrete volume is simple geometry: trench length multiplied by the 600mm standard width and the required depth. On normal ground that depth starts at 1m, and every extra centimetre buys more concrete at £105 – £140 per m³. Order 10% more than the calculated volume so a slightly over-dug trench does not leave you short partway through the pour.

Labour and plant are the next lines. The groundworker gang charges £200 – £280 per day, and a mini-digger at £125 – £250 per day does the excavation on most sites. Because a ready-mix truck rarely reaches a rear garden, most extensions also need a concrete line pump at £250 – £500 to move the pour from the road to the trench. The estimate then adds contractor overhead, spoil removal, and site mobilisation on top of the raw materials and labour.

When the ground changes the price

Ground condition is the single biggest swing in the total. On clay near mature trees, foundations commonly go far deeper to reach stable bearing strata below the zone where the soil shrinks and swells with the seasons. Deeper trenches mean more concrete at £105 – £140 per m³ and often shoring for safety. If good ground sits very deep, your structural engineer may specify mini-piling instead, which adds £5,000 – £15,000 on top of a normal foundation budget.

Rock is the other surprise. The digger needs a breaker attachment at £200 – £400 per day, and a dig that would take a day in normal ground can stretch to two or three. Access matters as well: where even a micro-digger cannot fit through the side return, the trench has to be dug by hand, and the £1,000 – £3,000 hand-dig premium reflects how slow and punishing that work is.

A worked example

Take a mid-sized rear extension with around 18 metres of external trench on normal ground. At 600mm wide and 1m deep, that gives the base concrete volume shown above, which you multiply by £105 – £140 per m³ and then top up by 10% for over-order. The groundworker gang works at £200 – £280 per day, the mini-digger at £125 – £250 per day, and a line pump at £250 – £500 carries the concrete to the trench. Add contractor overhead and the calculator returns the total range at the top of the page.

Then hold a buffer. A documented Oxfordshire extension found its trenches had to go deeper than the first estimate assumed, which is exactly why 20% contingency on the groundwork phase is the sensible minimum: the bottom of the trench is the most common place for a foundation budget to blow out. No concrete goes into the ground until building control has inspected the open trench, so build that inspection into your programme too.