A basement under a terraced house in London

A basement extension application in a London borough turns on two documents: the impact assessment that proves the ground and groundwater can take the excavation, and the construction management plan that proves the street can survive the lorries. The scheme below is illustrative, built from the pattern that repeats across boroughs with an adopted basement policy, and it shows where the work and the money actually went.

The house is a mid-terrace of three storeys, brick, around 1890, in a conservation area, with a 14 m rear garden. The owners wanted a single-storey basement under the full footprint of the house with a light well at the front and a rear extension under the garden, giving a family room, a plant room and a utility.

The house and what was proposed

Two things about the house drove everything else. It is mid-terrace, so both flank walls are party walls, and the ground is London Clay over terrace gravels with a perched water table the ground investigation later found at just under 3 m.

The first sketch showed the basement extending 6 m into the garden. That was reduced twice.

The borough policy and what it restricted

Boroughs that have adopted a basements policy tend to control the same variables: how many storeys below ground, what proportion of the garden can be built under, whether the basement can extend beyond the footprint of the host dwelling at all, how much soil depth has to remain above any structure under a garden, and whether the site sits in a flood zone or over a sensitive aquifer. Wording differs, so read the adopted policy and any supplementary planning document together rather than assuming a neighbouring borough's rules apply.

In this case the policy capped the garden incursion at half the garden area and required 1 m of soil above any part of the basement under the garden for planting. The 6 m projection became 3.5 m, and the flat roof of the garden section became a planted build-up with a drainage layer, which pushed the internal floor level down and the excavation deeper.

That is the sequence to expect. A policy limit on plan area frequently becomes a depth problem, and a depth problem becomes a groundwater problem.

The basement impact assessment scope

The Basement Impact Assessment followed the staged structure most London boroughs ask for: screening, scoping, site investigation and study, impact assessment, and a non-technical summary a neighbour can read. Screening asks a set of yes or no questions about surface water, groundwater and land stability. Any yes carries forward into scoping, which decides what has to be investigated.

Here the screening flagged three matters. The site is within a wider area of surface water flood risk, the excavation would intersect the gravels, and there are structures within the zone of influence on both sides and behind.

Scoping turned those into a specific brief: two boreholes with standpipes monitored over eight weeks, a slope stability check given the 1.2 m fall across the plot, a ground movement assessment to predict settlement at the neighbouring party walls, and a surface water drainage calculation for the changed roof and hardstanding areas.

Most boroughs with a basements policy also require the assessment to be independently audited by an engineer appointed by the council, with the applicant paying the audit fee. Budget for it, and expect the auditor to ask for one round of clarification. The audit is where a thin assessment becomes an expensive one.

Groundwater and structural evidence

The eight weeks of standpipe monitoring mattered more than anything else in the file. It established a groundwater level, a seasonal range and a reason to design a drained cavity system rather than fight the water with a fully tanked box.

The ground movement assessment predicted damage at Burland category 0 to 1 at the two adjoining properties, which is the aesthetic end of the scale: hairline cracking, redecoration. That prediction is what allowed the officer to conclude there was no unacceptable effect on neighbouring structures. Had it come out higher, the underpinning sequence, the depth or the plan area would have had to change.

A Structural Survey of the host dwelling and a condition survey of both neighbours went in alongside. The condition surveys are not a planning requirement in most boroughs, but they protect the owner later, and they answer the objection that the neighbours' houses were already cracked before anyone dug anything.

Structural and geotechnical documents of this kind need a chartered engineer. PlanScope can draft the planning narrative around them, and for specialist technical reports, qualified professional input may be required.

Construction traffic and neighbour objections

Twenty-one objections came in during the publicity period. Almost all of them were about lorries, skips, noise hours and where the spoil was going, and only two were about the principle of a basement.

That distribution is normal, and it tells you where to put the effort. A basement under a 60 m² footprint at 3 m depth is around 180 m³ of spoil before you add the garden section, and on a street with no off-street parking and a controlled parking zone, every cubic metre leaves in a lorry that has to stop somewhere legal.

The Construction Management Plan answered it with specifics rather than reassurance: a single loading position with a suspended bay licence, a banksman present for every delivery, deliveries confined to between 9.30am and 3.30pm to avoid school times, a conveyor rather than barrows for spoil removal, wheel washing, a named site contact, and a commitment to the Considerate Constructors Scheme. Noise and vibration limits were set against BS 5228, and the plan committed to monitoring at the party walls.

The conditions that followed

Permission came with conditions the owners had effectively drafted themselves, which is the point of submitting a detailed plan early. The construction management plan was made a condition in its submitted form. Approval of the basement construction method and the movement monitoring regime was reserved as a pre-commencement condition. Planting on the garden roof had to be implemented in the first planting season after completion, with replacement of failures for five years.

One condition was unwelcome: a restriction on piling method requiring written approval before any rig arrived on site. That took a further application to discharge, and a further eight weeks. Read the walkthrough of a basement under a Victorian terrace for a comparison where the conditions landed differently.

Cost and programme against the first estimate

The owners had budgeted for the design team and the build, and had not budgeted for the evidence base. Ground investigation, eight weeks of monitoring, the impact assessment, the audit fee, the movement assessment, the condition surveys and the traffic work together formed a line item nobody had priced at the start.

The programme slipped by about four months against the original plan, and almost all of it was the monitoring period plus the audit round trip. The build itself started roughly when the contractor had said it would once permission was in place.

If you take one thing from this: commission the ground investigation and start the standpipe monitoring before the architect finalises levels. The water level determines the waterproofing strategy, the waterproofing strategy determines the floor build-up, and the floor build-up determines whether your head height works. Doing it the other way round is how a scheme gets redesigned twice.

For a wider view of which documents a below-ground scheme triggers, see basement extensions under an existing house, or enter your postcode on PlanScope to see which reports your site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.