Basement Impact Assessment (BIA)
An assessment of how a new basement will affect ground stability, groundwater, drainage and neighbouring buildings, as many London councils require for planning.
About this service
A Basement Impact Assessment, usually shortened to BIA, explains to the council how a proposed basement will affect the ground, groundwater, surface water and neighbouring buildings. Several London boroughs require one with any planning application that includes a new or extended basement, and the planning officer will not decide the application without it. Our BIA sets out the evidence, the assessment and the mitigation in the format your council expects.
Most of our BIA clients are London homeowners adding a basement under a house or garden, along with architects and developers working on terraced and semi-detached sites. Requirements vary by borough. Camden and Kensington and Chelsea have detailed basement policies, and other councils increasingly ask for similar information. Our London basement guide covers the wider process from feasibility to construction.
Basements in London are dug into London Clay, river terrace gravels or made ground, often right against party walls and sometimes near the routes of buried rivers. Excavation can cause movement in neighbouring houses, and a basement can block or divert shallow groundwater. A well-prepared BIA shows the council these risks are understood and controlled.
When you need it
- You're applying for planning permission for a basement
- Your council's basement policy requires a BIA
- The council has asked for more information on a basement
What's included
- Screening and scoping to council guidance
- Review of geology, hydrology and site history
- Assessment of land stability and groundwater effects
- Impact on neighbours and mitigation
How it works
- ScreeningWe answer the council's screening questions on groundwater, land stability and surface water, using geological maps, historical records and site information to identify which issues need further assessment.
- ScopingFor each issue screened in, we define the further information or analysis needed, such as site investigation, groundwater monitoring or a ground movement assessment, and agree the scope with you.
- Site investigation reviewWe review borehole and trial pit results, groundwater levels and soil test data. If fieldwork has not been done yet, we can arrange it through a specialist contractor.
- Impact assessmentWe assess effects on land stability, groundwater and drainage, estimate ground movements and the likely damage category for neighbouring buildings, and consider the cumulative effect of nearby basements.
- Mitigation and reportWe set out mitigation such as construction sequence, temporary propping and drainage measures, compile the BIA for submission and respond to queries from the council or its auditor.
What you receive
What councils and auditors look for in a BIA
Some councils send BIAs to an independent auditor, and the applicant usually pays for that review. Auditors check that the screening is honest, that ground data is site-specific rather than assumed, and that predicted movements to neighbours are realistic. The commonest reasons for queries are missing groundwater monitoring, a construction sequence that does not match the structural design, and damage assessments that ignore party walls or rear additions.
Councils generally expect predicted damage to neighbouring buildings to be no worse than very slight on the recognised damage category scale, though the exact threshold is set by local policy and the planning officer will confirm it. Where early figures are too high, the usual fixes are stiffer temporary propping, a different underpinning sequence or a piled wall. Our ground movement and damage assessment provides the detailed analysis when the BIA needs it.
A BIA is a planning document, not a construction design. Once permission is granted, you will still need full structural design, temporary works and party wall agreements before excavation starts. Keeping the BIA, the structural method statement and the party wall process consistent saves time later, because the neighbours' surveyors will often ask for the same information again.
Not included
- Site investigation fieldwork, which can be arranged
Projects that used this service
Frequently asked questions
What is a Basement Impact Assessment?
It is a report submitted with a planning application that assesses how a new or enlarged basement will affect ground stability, groundwater, surface water drainage and neighbouring buildings. It follows staged screening, scoping, site investigation, impact assessment and mitigation. Councils with basement policies use it to decide whether the basement can be built without unacceptable harm to the area or adjoining properties.
Which London councils require a BIA?
Requirements change and differ between boroughs. Camden, Kensington and Chelsea and Westminster are well known for detailed basement policies, and other councils ask for similar assessments, sometimes under a different name. The safest approach is to check your council's local plan and basement guidance, or ask at pre-application stage. We prepare the assessment to suit the relevant council's guidance.
Do I need a site investigation for a BIA?
In most cases, yes. Councils and auditors expect site-specific information on soil and groundwater rather than desk-based assumptions. That usually means at least one borehole near the basement with groundwater monitoring over a period, plus trial pits to expose neighbouring foundations. We can arrange the fieldwork through a specialist contractor if it has not already been done.
Is a BIA needed for a lightwell or a small basement extension?
Often it is. Many basement policies apply to any new or extended basement, including front lightwells and extensions under a garden, not only full basements under the house. Some councils accept a shorter assessment where screening shows little risk. Ask the council early, or send us the drawings and we will tell you what the relevant policy is likely to need.
How much does a Basement Impact Assessment cost?
Our Basement Impact Assessment starts from £2,000 + VAT. The fee depends on the council's requirements, the complexity of the site and how much detailed analysis is needed. Site investigation fieldwork is separate and can be arranged, and any audit fee charged by the council is also separate. We confirm a fixed fee in writing before work starts.