What is included in a Basement Impact Assessment?
A Basement Impact Assessment is a multi-discipline technical report that assesses the risks a basement development poses to neighbouring properties, groundwater, drainage and trees. A standard BIA for a London residential basement includes: (1) Ground investigation — desk study using BGS borehole records plus site investigation (typically 2–4 trial pits or boreholes to 6m depth) to confirm soil type, ground conditions and groundwater depth; (2) Structural impact assessment — review of neighbouring building foundations and party walls, assessment of differential settlement risk during and after construction; (3) Hydrogeological assessment — analysis of groundwater flow and levels, assessment of the risk that a waterproof basement structure will impound groundwater and raise levels upstream; (4) Tree survey — identification of any trees with TPOs or covered by a conservation area, root protection area mapping, assessment of the basement footprint within root zones; (5) Construction methodology statement — describes how the basement will be built (underpinning sequence, temporary works, propping), the programme of works and any monitoring proposed. The final report is submitted as a supporting document with the planning application.
BIA cost breakdown by element
A BIA has several discrete cost components. Desk study and report structure: £500–£1,500 (specialist basement engineer's time). Ground investigation (trial pits/boreholes): £1,500–£4,000 depending on number and depth — 2 trial pits for a small cellar conversion vs 4+ boreholes to 10m for a double-storey sub-garden basement. Structural impact assessment: £800–£2,000. Hydrogeological assessment: £800–£3,000 — more expensive on Thames gravel terrace sites (SW/SE London) and in known high groundwater areas. Tree survey (if required): £500–£1,500. Construction methodology statement: £600–£1,500. Total for a simple cellar conversion BIA: £4,000–£5,500. Total for a full double-storey basement extension BIA: £8,000–£12,000. Some specialist basement contractors include BIA procurement in their project management fee — Builderr co-ordinates BIA preparation as part of the pre-planning service.
Which London boroughs require a BIA?
All 33 London boroughs will typically require some form of technical impact assessment for basement planning applications, but the formal BIA requirement — as named in adopted planning policy or SPD — applies most stringently in: Kensington and Chelsea (mandatory); Camden (mandatory, specific format required); Westminster (mandatory for all basement applications); Hammersmith and Fulham (mandatory); Islington (required with any planning application involving basement work); Lambeth (expected in conservation areas and near the Thames); Southwark (expected near the Thames terraces). Other boroughs apply national policy (NPPF) — a BIA or equivalent technical note is expected on any application that could affect neighbouring structures or the water environment. There is no London-wide exemption for small projects.
When is a BIA not required?
A BIA is not required for a pure internal cellar conversion that requires no planning permission — converting an existing basement void into habitable space without any excavation or external works. Building regulations approval is needed, but no BIA. A BIA is also generally not required for: above-ground extensions that do not involve any sub-ground works; loft conversions with no basement component; garage conversions at ground level. If you are uncertain whether your project triggers a BIA requirement, a pre-application enquiry to the local planning authority (costing £100–£500 depending on borough) will confirm the requirement within 4–6 weeks. Builderr includes pre-application strategy in all basement project scope assessments.
