Ask ten builders how deep your extension footings need to be and you will hear everything from 600mm to well over a metre. The honest answer is that depth is decided by the ground you are building on, not a number in a book. Here is how it actually works, based on what we dig week in, week out around Aberdare and the wider Cynon Valley.
For most single storey extensions, footings end up between 750mm and 1 metre deep. Building regulations set a minimum of 450mm to get below frost level, but in practice almost no building control officer will pass a trench that shallow because the ground near the surface is rarely firm enough. One metre to the bottom of the trench is the working rule of thumb on typical sites, and it is the depth we price for unless a survey tells us otherwise.
The key point is that the final depth is confirmed on site, in the open trench, by your local authority building control inspector or an approved inspector. They want to see the bottom of the excavation sitting on firm, undisturbed natural ground. If it is not there at 900mm, you keep digging until it is.
Much of the housing around Aberdare, Mountain Ash and Abercynon sits on a mix of glacial till, clay and made ground on the valley sides. Clay shrinks in dry summers and swells when wet, so footings in clay generally need to go to at least 900mm to 1 metre to get below the zone where that movement happens. Shallower than that and the footing can lift and drop with the seasons, which is where cracked render and stepped cracking in brickwork come from.
This is also a former mining area, so pockets of fill, old spoil and disturbed ground are common, particularly on terraced streets where gardens were levelled generations ago. If we open a trench and find soft fill, ash or old rubble, the footing has to go through it to solid ground beneath. On sloping plots, which the valley has plenty of, footings are often stepped so each section still reaches the required depth into the bank.
Trees are the single biggest reason footings go deeper than expected. Roots draw moisture out of clay, so a footing near a tree, or near one removed in the last few years, may need to go to 1.5m, 2m or beyond depending on the species and its distance from the trench. NHBC tables are the standard reference here, and a mature oak or willow within 10 metres can change the whole job. Flag any nearby trees before work is priced, not after the digger arrives.
Drains matter too. If a public sewer or your own drainage runs near the new wall, the footing usually has to be taken down to at least the level of the pipe invert so it does not load the pipe. Building over or within 3 metres of a public sewer also needs a build over agreement with Welsh Water, which is worth sorting early because it can hold a job up.
Depth gets all the attention, but width matters as well. A typical cavity wall extension needs a trench around 600mm wide, with the wall sitting centrally on the concrete. Most extensions round here use trench fill, where concrete is poured to within 150mm to 225mm of ground level, because it is quicker and safer than laying bricks at the bottom of a deep, wet trench. A GEN3 mix, roughly C20 strength, is the usual specification.
Whatever depth you end up at, the sequence is the same: dig, have building control inspect the open trench, then pour. Concreting before the inspection risks being told to expose or even break out the footing. In Rhondda Cynon Taf the council's building control team will come out at trench stage, and their word on depth is final. A sensible budget allows for going deeper than the drawing shows, because until the ground is open nobody can promise what is down there.
Yes. Almost every extension needs a building regulations application, and the footings must be inspected in the open trench before any concrete goes in. Your builder normally books this inspection with the council for you.
Deeper trenches mean more muck away and more concrete, and readymix is a significant part of the groundworks bill. As a rough guide, going from 1m to 1.5m deep can add several hundred pounds per side of the extension, though it depends on access and ground conditions.
Not reliably. Many older terraced houses in the valley sit on very shallow stone footings that would never pass today, and your extension must meet current standards regardless of what the original house was built on.
Tell us about the job and we'll give you a straight price, no messing.
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