London is not a harder place to dig because the soil is exotic. It is harder because the ground moves, the buildings are old and touching each other, and everything above the hole is regulated by somebody. Here is what that means in practice. The detail sits on the service pages below.
London Clay Moves
The majority of the capital is built on London Clay: a firm, workable soil with one significant vice. It is highly shrink-swell. It takes on water in winter and gives it up in dry summers, and the ground rises and falls with it. Foundations that are too shallow ride that movement, and the cracks show up in the brickwork above.
Not all of London behaves this way. The gravel terraces along the Thames (parts of west and east London in particular) are free-draining and comparatively stable, which changes what a foundation, a soakaway or a sub-base needs to do. A contractor who quotes the same depth everywhere in London has not looked at the ground.
Street Trees and Subsidence
The mature planes, limes and oaks lining London's Victorian and Edwardian streets are the classic subsidence driver. They draw moisture out of clay and shrink it, and NHBC guidance sets foundation depth accordingly based on species, distance and clay plasticity. This is exactly the scenario behind most of the underpinning work carried out in London, and it is why an honest quote asks what is growing near the wall before it commits to a depth.
The Party Wall etc. Act 1996
London is built shoulder to shoulder, so excavation next to a neighbour is the rule rather than the exception. Digging within 3m of an adjoining structure below the level of its foundations, or within 6m where a 45-degree line from that foundation would be cut, triggers a notifiable event under the Act. Extension foundations, underpinning and basements almost always qualify. Notices go to the adjoining owners before work starts, and excavation notices generally need two months. Building that into the programme early is the difference between a delayed start and a stopped job.
Access, Permits, CPZ and ULEZ
This is the part London quotes get wrong most often. A terraced street with no side access means every barrow of spoil and every tonne of aggregate comes through the house or the side return. A controlled parking zone means bays have to be suspended. A skip needs a permit from the borough. Muck-away lorries need somewhere to stand and a route wide enough to get there. Plant and vehicles have to be congestion charge and ULEZ compliant, and some boroughs restrict working hours on top.
None of that is unusual. All of it costs money and time. KiKK prices it at the site visit rather than discovering it in week two, which is why our quotes are sometimes not the cheapest sheet of paper you receive and almost always the one that turns out to be true.
Boroughs, Crossovers and TfL
Outside London, a dropped kerb goes through the county council. In London it goes through your individual borough (thirty-two of them, each with its own process, fees and timescales), and if the property fronts a red route or another TfL road, TfL consent is required as well. A new driveway in London is a highways application before it is a paving job.
SuDS, the London Plan and Old Drains
London wants water in the ground, not in the sewer. The London Plan pushes sustainable drainage hard, and nationally, laying more than 5m² of impermeable surface in a front garden that drains to the highway needs planning permission, a rule practically written for the London front garden. Meanwhile the network underneath is Victorian, frequently combined and often shared between properties, with clay pipes, open joints and root ingress. That is why CCTV drainage surveys in London are worth doing before anyone quotes a repair, and why permeable surfacing and soakaways are designed in rather than bolted on.
