Basement impact screening checklist for a terraced house

Before a council will look at a basement under a terraced house, most authorities with a basement policy want a screening exercise first: a short written check of whether the proposal could affect groundwater, surface water or the stability of the ground and the houses either side. Get the screening right and a modest single-storey basement under the existing footprint can often be signed off without commissioning monitoring wells and ground movement modelling. Get it wrong, or skip it, and the application stalls while an officer asks for the work you should have scoped at the start.

A basement impact screening checklist is not a substitute for the assessment itself. It decides how much assessment you need.

Screening questions the authority will ask

Councils that have adopted a basement policy usually structure screening around three headings: subterranean water flow, surface water and flooding, and land stability. The questions under each are close to yes or no, and each yes pushes you towards more detailed work.

The practical list for a mid-terrace house looks like this:

1. Does the proposal extend beyond the footprint of the existing house, under the garden or under the pavement?
2. Does it go deeper than one storey below existing ground level?
3. Is the site on a slope, or is there a change in level across the terrace?
4. Is there a watercourse, culvert, former watercourse or pond within the locality?
5. Does the site sit on a permeable stratum overlying clay, where a perched water table can form?
6. Is the house within a flood zone, or in an area shown as at risk from surface water?
7. Are there trees of any size within a distance that could be affected by the excavation?
8. Will the shared party walls need support during the excavation?

For a terrace the answer to the last one is almost always yes, which is why party wall and movement issues dominate the response even where the hydrology is straightforward.

Ground conditions and the desk study sources

Screening is a desk exercise, and the desk sources are free. The British Geological Survey mapping gives the superficial deposits and bedrock at the address, which tells you whether you are dealing with London Clay, river terrace gravels, made ground or something more awkward. The council's own local plan evidence base often contains a strategic flood risk assessment with maps of groundwater emergence and historic watercourses.

Record what you found and where you found it. A screening report that says "the site is underlain by clay" without naming the source is the single most common reason for a request for further information.

Old maps matter as much as geology. A terrace built over a filled-in stream, an old brick pit or a bomb-damaged plot behaves nothing like its neighbours, and that history will not appear on a modern survey. Where the desk study throws up made ground of unknown depth, expect a geotechnical investigation to be asked for, and read the Geotechnical Assessment page for what that involves.

Surface water and groundwater flow paths

Two separate questions hide inside "water". The first is whether the basement obstructs groundwater moving through the ground. The second is whether the basement changes where rainwater goes on the surface.

Groundwater obstruction matters where the basement box sits within a water-bearing layer. In a gravel over clay sequence the basement can dam the flow and push water sideways into a neighbour's cellar, which is the outcome basement policies exist to prevent. Where the whole excavation sits within impermeable clay well above any water table, the honest screening answer is that the flow question does not arise, and you say so in one sentence with the borehole evidence behind it.

Surface water is more mundane and more often mishandled. A basement usually comes with a light well, a new rear extension over it, or a garden lowered to make the space work, and the run-off from all of that has to go somewhere. Check the Environment Agency flood map for planning and the surface water risk mapping, then show where the water will drain. The arithmetic is the same as for any small site, and the method in a worked greenfield run-off rate for a 0.6 hectare site scales down.

If any part of the garden is being paved or covered, deal with it in the screening rather than leaving it for the drainage officer. A drainage strategy for a minor residential scheme shows the level of detail councils accept on small schemes.

Neighbouring structures and movement risk

The party walls of a Victorian terrace usually sit on a shallow brick footing, sometimes no more than 300 mm wide, with no foundation to speak of. Supporting them in short sequenced bays is routine work for a contractor who does it every week, and it is also the part of the job most likely to crack a neighbour's plaster.

Screening needs to record three things: the form and depth of the existing foundations where known, the proposed method of support, and the movement that method is expected to cause. A structural engineer will put a predicted movement figure and a damage category against it. Where the prediction is more than very slight cosmetic damage, a full basement impact assessment with a ground movement analysis follows.

Party wall procedure sits outside the planning system entirely. Serving notice under the Party Wall etc. Act 1996 and agreeing a schedule of condition is a civil obligation that runs in parallel, and a council cannot waive it or enforce it. Homeowners regularly discover this at the worst possible moment.

Where the neighbouring house is listed, or the terrace sits in a conservation area, the calculation changes again because the risk of harm to a heritage asset is a planning matter in its own right.

Trees and their water demand

Trees near a basement create two problems that pull in opposite directions. Removing a mature tree from a clay site allows the clay to swell as it recovers moisture, lifting foundations over years. Keeping the tree means excavating within its root protection area.

Screening should record every tree within influencing distance, including trees in neighbouring gardens and street trees, with species and rough stem diameter. Species matters because water demand differs sharply: a poplar or willow on clay is a different proposition from a birch. Where a tree is protected by a Tree Preservation Order or stands in a conservation area, the works need separate consent and the arboricultural method statement becomes part of the application rather than an afterthought.

When a full assessment becomes unavoidable

Any one of the following usually takes a scheme from screening to full assessment:

• Excavation extending beyond the original footprint, under a garden or beneath the highway.
• More than one storey of basement, or a depth that reaches a water-bearing layer.
• A predicted damage category above very slight, or a neighbouring structure in poor condition.
• A site within Flood Zone 3, or where the basement will contain a bedroom.
• Made ground, a former watercourse or contamination indicated by the desk study.

At that point the work belongs to qualified people. Councils expect a Basement Impact Assessment to be written by a chartered engineer with relevant hydrogeology and geotechnical input, and professional review is strongly advised even where the policy does not name a competence standard.

Putting the screening submission together

Keep it short and evidenced. Five or six pages covering the three screening headings, a table of the yes or no answers with the source of each, a drawing showing the proposed extent against the existing footprint, and a clear statement of which further work you accept is needed.

State the negatives explicitly. An officer reading "no watercourse is recorded within 100 metres on the historic mapping consulted" can close that question. An officer reading silence cannot, and will ask.

Illustrative example, and it is illustrative only: a mid-terrace house on London Clay, single-storey basement within the existing footprint, one light well at the front, two small garden trees, no flood zone. The screening report runs to six pages, answers every question with a named source, records a predicted damage category of very slight with a schedule of condition to follow, and concludes that no ground movement modelling is needed. The council validates it and asks only for confirmation of the light well drainage.

You can draft the supporting narrative for a basement application in PlanScope and send it for professional review before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.