There is no single depth written into the rules for extension footings, which surprises most homeowners. Building Regulations set out what the foundation has to achieve, and the actual depth is decided on site by the building control inspector once the trench is open and the ground can be seen. Around Watford and the rest of south west Hertfordshire that usually means deeper than people expect, because so much of the area sits on clay.
For a standard single or two storey domestic extension, most builders and inspectors work from a minimum trench depth of 1 metre below finished ground level. Approved Document A of the Building Regulations gives guidance on strip foundation widths for different wall loads and soil types, but it deliberately does not fix a depth, because the ground varies so much from one street to the next.
On good, undisturbed ground such as chalk or dense gravel, an inspector may accept somewhat less than 1 metre, though 750mm is about as shallow as you will realistically see for anything more than a small porch. On clay, 1 metre is the floor, not the ceiling. Strip foundations for a brick and block cavity wall are usually 600mm wide and filled with at least 225mm of concrete, though many trenches are simply mass filled to within a few courses of ground level so the bricklayer starts on a level base.
Watford, Bushey, Rickmansworth and much of the surrounding area sit on London Clay or clay with flints. Clay shrinks when it dries and swells when it gets wet, and that movement is greatest near the surface. A footing needs to sit below the zone where seasonal moisture change happens, otherwise the extension will move differently to the house and you get cracking at the junction.
The NHBC guidance most inspectors lean on classifies clay by how much it moves. High shrinkage clay with no trees nearby usually requires a minimum of 1 metre. Add trees or hedges and that depth climbs fast, because roots draw moisture out of the ground to a much greater depth. It is not unusual for a Watford extension near a mature oak, willow or poplar to need trenches of 2 metres or more, at which point a trench fill foundation becomes expensive and a piled or pad and beam solution may work out cheaper.
Recently removed trees matter just as much. Clay that was dried out by roots will slowly rehydrate and heave upwards for years afterwards, so the inspector may still insist on deeper footings and a compressible void former against the inner face of the trench.
The sequence is straightforward. The trench is dug to the expected depth, the bricklayer or groundworker checks the base is level and free of loose material, and then building control is called to inspect before any concrete goes in. This is the point where the depth is confirmed or changed. If the inspector sees soft spots, water, old drains or roots at the bottom of the trench, they will ask you to go deeper or step the foundation, and you must not pour until they are happy.
Around Watford you will also frequently hit old ground. Victorian and inter war housing often has buried rubble, filled cellars, soakaways or the remains of old outbuildings within the footprint of a new extension. Footings have to reach undisturbed natural ground beneath any of that, which is another reason a quoted depth can change once digging starts.
Existing drains and the neighbour's foundations affect depth too. If a public sewer runs within 3 metres of the new footing, Thames Water will usually require the foundation to go down to at least the invert level of the pipe, or to be bridged with a lintel. Where the extension footing sits next to a shallower neighbouring foundation, it may need to be stepped down gradually to avoid undermining it, and the Party Wall Act notice becomes relevant if you are digging within 3 metres and below their foundation level.
Foundation depth is one of the biggest variables in an extension quote, so treat any figure given before the trench is open as an estimate. A 1 metre trench fill footing for a typical 4 by 5 metre rear extension might use in the region of 10 to 14 cubic metres of ready mix concrete. Every extra 300mm of depth adds roughly 3 to 4 cubic metres, plus the additional muck away, which in Hertfordshire currently runs to several hundred pounds per lorry load.
If your garden has large trees or your neighbours have already needed deep footings, it is worth paying for a soil investigation or at least a trial hole before you commit to a design. A few hundred pounds spent early can avoid a mid project surprise running into thousands. A good bricklayer or groundworker will be honest that the final depth is the inspector's call, and will price the concrete as a provisional sum rather than a fixed figure.
You can, but the trench must be safe, square and taken to whatever depth building control asks for, and you are responsible if it collapses or needs redigging. Anything beyond about 1.2 metres deep should be shored or battered back, so most people leave it to a groundworker.
Yes. The load is lighter, but the depth is driven by the ground, not the height of the wall. On clay a single storey extension near trees needs the same depth as a two storey one.
No. Older houses in Watford often have footings of only 300 to 450mm, which would not be accepted today. The new footing is designed to current standards, and the junction between old and new is detailed to allow for slight differential movement.