Reports for a basement under a terraced house

Digging a basement under a terraced house is the most technically demanding thing a householder can apply for, and the document list reflects that. On a terrace you are excavating between two buildings that rely on shallow Victorian footings, usually with no side access, often on clay that shrinks and swells, and the council will want evidence on ground stability, groundwater, surface water, construction traffic and neighbour protection before it grants anything.

The controlling document is normally a basement impact assessment, and the authorities that ask for one set out in their own policy what it must contain. That policy is where to start, not the national framework.

Where basement policy comes from

There is no national planning policy on residential basements. The rules come from local plan policies and supplementary planning documents, and they vary sharply.

Several London boroughs have detailed basement policies, developed after a decade of disputes, which limit basements to a single storey, restrict how far they may extend under a garden, require a minimum depth of soil over the roof of any garden element for planting, and require the impact assessment to be independently audited at the applicant's expense. Outside London, some authorities have adopted similar policies and many have none at all, in which case the application is assessed on general amenity, drainage and stability grounds.

Read the adopted policy and any basement guidance before the design is fixed. The garden extent limit in particular is the constraint that kills schemes late, because it determines the floor area and therefore the viability of the whole project.

Ground and groundwater investigation

The assessment works through three subjects: land stability, hydrogeology, which is groundwater, and hydrology, which is surface water and flooding. Each is screened, then scoped, then investigated if the screening does not rule it out.

Screening is a desk exercise against published data, and for a terraced house in a city it rarely rules everything out. Expect to carry out a ground investigation: boreholes or window samples to below the proposed formation level, standpipes left in for groundwater monitoring over a period, and laboratory testing of the soils.

Two points make the difference between a report that satisfies the council and one that comes back.

Monitoring period. A single groundwater reading taken in August tells nobody anything about winter levels. Where groundwater is a live issue, monitoring needs to run for months, and that has to be programmed before the application, not after.

Depth of investigation. Boreholes stopped at formation level do not answer questions about what happens below the base slab. The geotechnical assessment should go deep enough to characterise the strata influencing settlement and heave.

Perched water in made ground is the specific hazard in older terraced streets, where infilled cellars, old wells and redundant drains create pathways nobody has recorded.

Movement assessment and the neighbours' walls

The structural question is not whether your basement will stand up. It is how much the houses either side will move while you build it.

The engineer models the ground movement from the excavation and from any underpinning sequence, then predicts the resulting strain on the neighbouring buildings using the standard damage classification, which runs from negligible through very slight, slight, moderate and upwards. Many authorities expect the predicted damage to neighbouring properties to be no worse than the very slight category, and a prediction above that is where objections gain traction.

The assessment has to reflect the construction sequence actually proposed. Hit and miss underpinning in one-metre bays, with a stated maximum number of open bays at any time, produces a different prediction from a contiguous piled wall, and a report that models an idealised sequence the contractor will not follow is worth little.

Monitoring is the other half. A movement monitoring regime, with agreed trigger levels and stated actions when a trigger is reached, is what allows an officer to approve something that could go wrong. Offer it in the application rather than being asked.

The Party Wall Act runs in parallel

Planning permission and party wall procedure are separate regimes, and neither substitutes for the other.

Excavating within three metres of a neighbouring structure to a depth below its foundations, or within six metres where a line drawn down at 45 degrees from the base of their foundations would meet your excavation, requires notice under the Party Wall etc. Act 1996. So does work to the party wall itself, which on a terrace is unavoidable.

Serve notices early. Appointing surveyors, agreeing an award and carrying out a schedule of condition on both neighbouring properties takes months, and the award will often specify monitoring and working methods that feed back into the design. Party wall matters are a private law process, so they are not something PlanScope produces reports for, and they are not something the council will resolve for you.

Neighbours who feel properly informed object less. A schedule of condition and a clear explanation of the sequence does more to prevent objections than any amount of policy argument.

Construction traffic and protecting the street

A basement under a terrace generates lorry movements out of all proportion to its size. Several hundred tonnes of spoil leave the site, concrete arrives, and there is nowhere to put any of it.

Councils respond by requiring a construction management plan, and on constrained streets a Construction Traffic Management Plan dealing with routing, hours, wheel washing, skip and grab lorry positions, and how the footway is protected. Where the street is narrow or on a bus route, the highway authority will have views on holding positions and banksman arrangements.

Set out the working hours you propose and the total number of expected heavy vehicle movements. Vague plans attract conditions that are stricter than anything you would have proposed yourself.

Surface water, drainage and flooding

A basement changes the ground's behaviour. It displaces groundwater flow, it removes storage in permeable soils, and it creates a space below the level of the drainage system.

The application needs to show where surface water goes, how any below-ground drainage is pumped and what happens in a power cut, and whether the basement lies in an area at risk of surface water or groundwater flooding. Non-return valves, a sump with dual pumps and a battery backup, and a raised threshold at any external lightwell are the standard answers.

Where the street has a history of surface water flooding, expect the lead local flood authority or the council's drainage engineer to look closely at whether the scheme moves water onto neighbours.

Trees, lightwells and what gets conditioned

Basements under rear gardens meet tree roots. A survey to BS 5837:2012 establishes root protection areas, and an excavation inside one usually has to be redesigned rather than mitigated.

Lightwells in front gardens are a conservation area issue, since they alter the relationship between the house and the street. Railings, grilles and their detailing become a design matter rather than a technical one.

Expect conditions requiring the approved construction sequence to be followed, the monitoring regime to be implemented with results submitted, hours of work, and no deviation from the approved drawings without agreement. Where an independent audit of the impact assessment is required, budget for it and for the response, and the article on responding to a basement impact assessment audit sets out how that exchange usually runs.

The full document list, including the basement impact assessment itself, is set out in the article on the reports needed for a basement excavation.

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.