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Start with the ground beneath your feet

Every extension conversation eventually arrives at the same question: what are we actually building it on? The foundation is the one part of the job you cannot sensibly revisit later, so it is worth spending a little time and a modest amount of money investigating the ground before anyone orders concrete. Your structural engineer and building control body will want evidence rather than assumptions, and a couple of trial holes are far cheaper than a failed inspection or, worse, movement in five years' time.

For most single-storey domestic extensions in the UK, the choice comes down to three options: trench fill, pad or raft. Each suits different ground conditions, different loads and different budgets. Here is how to work out which one your project is likely to need.

What the soil is telling you

Before choosing anything, you need to know what you are building on. In much of southern and central England you will meet shrinkable clay, which swells in winter and shrinks in summer. In parts of the Midlands, Wales and the north you may find sand, gravel, chalk or rock, all of which behave very differently. Made ground, old foundations, infilled ponds and former quarrying are common in urban areas and can catch you out.

A useful first step is a couple of trial holes dug by hand or mini-digger to around 1.2 metres, plus a dynamic probe test if the ground looks variable. Things to establish:

• The depth of topsoil and any soft or made ground
• The type of subsoil and whether it is firm, loose or waterlogged
• The presence of trees or hedges within influencing distance — roots dry clay out and cause heave
• Any drains, old soakaways or services crossing the footprint

As a rule of thumb, foundations on good firm subsoil should be at least 1 metre deep to get below the zone affected by seasonal moisture change. Near a mature tree, that depth can increase substantially depending on the species and its distance from the extension.

Trench fill foundations: the dependable default

Trench fill is the workhorse of UK extensions and what most builders will quote for first. A trench, typically 600mm wide, is excavated to the required depth and filled almost to the top with concrete, leaving around 150mm for the oversite and floor build-up. There is little or no brickwork below ground, which means no deep excavation to work in and no bricklayer waiting at the bottom of a hole.

It suits firm, stable subsoil and extensions with reasonably heavy walls — blockwork, brick or rendered masonry under a tiled or slate roof. It is quick, robust and well understood by building control. The trade-offs are the volume of concrete required and the fact that it becomes expensive fast if the trench needs to go very deep for trees or soft spots.

Pad foundations: lighter loads, fewer trenches

If your extension is timber-framed, steel-framed or a lightweight garden room, pad foundations can be far more economical than running a continuous trench around the whole perimeter. Instead, you excavate isolated square pads, usually 600mm to 900mm across, at each column or post position.

Pads are ideal where:

• The superstructure is carried on posts or a frame rather than solid walls
• Ground conditions are reasonable but you want to disturb less of the site
• There are existing drains or services you would rather not straddle with a trench

The pads still need to be founded at similar depths to a trench fill, and they rely on a properly designed ground beam or ring beam to tie them together and spread the load. Get the reinforcement and beam detailing right and this is a very efficient solution.

Raft foundations: spreading the load when ground is weak

A raft is a reinforced concrete slab, usually 150mm to 200mm thick with thickened edges, that spreads the entire weight of the extension across a wide area. It is the go-to solution where the ground is soft, variable, or where shrinkable clay and nearby trees make deep traditional foundations impractical or uneconomic.

Rafts also come into their own on sites with a history of mining subsidence or where a traditional foundation would need to be so deep that the cost becomes unreasonable. They are more involved to build — the reinforcement must be correctly placed and the concrete carefully poured and cured — so expect a higher specification and closer supervision.

Party walls, boundaries and the neighbours

If your extension is anywhere near a shared wall or boundary, the Party Wall etc. Act 1996 is likely to apply. You must give adjoining owners at least two months' written notice before starting, and the rules are particularly relevant to foundations.

Under the Act, you generally need to notify a neighbour if you intend to excavate within 3 metres of their property and deeper than their existing foundations, or within 6 metres if your excavation goes deeper than theirs by a specified amount. This is designed to protect their building from your digging. A party wall award will usually set out how the excavation is to be carried out and who pays for any preventative works.

Two other practical points worth checking early:

• Drains: if a public sewer runs within the footprint, you may need a build-over agreement before any concrete is poured
• Boundary ownership: confirm exactly where the boundary sits so footings do not stray onto a neighbour's land

Budget, sequencing and getting the details right

As a rough guide, trench fill is usually the cheapest per linear metre on good ground, pads can work out cheaper for framed structures, and rafts carry the highest upfront cost but can save money overall on difficult sites. None of these figures mean much without a site-specific design, though.

Whichever route you take, book your building control inspections at the right stages: before pouring concrete, after reinforcement is in place, and before the floor is covered. Keep photographs of the excavation and steel. If the ground turns out to be worse than expected, stop and speak to your engineer rather than pressing on — a short delay now is far cheaper than underpinning later.