Two Storey Extension Foundations and Structure

How the weight gets to the ground

The roof and the first floor rest on the walls or frame of the extension, and the walls pass that weight down to new foundations. Where the extension meets the house, the old rear wall usually comes out at ground floor level to make one big room. A steel beam then carries the house wall above, sitting on padstones at each end.

The engineer's calculations cover all of it: foundation width and depth, beam sizes, how the first floor joists span, and how the frame resists wind. Building control checks them against Approved Document A. A two storey scheme typically costs £800 to £1,800 in engineer's fees, and a standard beam supplied and fitted about £1,200 to £2,500.

Which foundation you are likely to need

On firm ground, a concrete strip or trench fill foundation is normal, and it is the cheapest. Clay is where things change. Clay shrinks and swells with moisture, and trees and hedges dry it out, so foundations near them go deeper to reach stable ground.

When the ground is poor, the trenches would get too deep or the site is made ground, the engineer may call for piles with concrete ground beams on top. They are sized from a ground investigation, and they can put several thousand pounds on the bill. No honest national figure exists, because how many and how deep depends wholly on your ground.

Trial holes dug before the price is fixed are the best protection against a surprise on the day the digger arrives. Building control also inspects the trenches before the concrete goes in.

What a prefab frame needs from its base

Timber frame and SIPs panels are lighter than block walls, but the foundations are still designed for two floors, furniture, people and snow on the roof. The difference with prefab is accuracy. Panels and modules are made to the millimetre in a factory, so the base they land on has to be level and square to tight tolerances. Most firms either build the base themselves or send a setting-out drawing and check it before the lorry leaves.

Volumetric modules, finished boxes for each floor, often sit on ground beams or piles rather than a full slab. Lifting either kind needs a crane with firm standing and enough reach; budget about £1,400 for a day's contract lift, meaning a 25 t crane and the crew who plan and sling each load.

Ground and foundation at a glance

A general guide. Your engineer designs the foundations for your ground and your extension.