Basement Impact Assessments that Get Your Site Approved and Built

Why do I need a Basement Impact Assessment?

All Local Authorities within Greater London, and many beyond, now require a Basement Impact Assessment (BIA) to support planning applications involving any form of subterranean development.

This could include a basement for a new development, a basement under an existing dwelling, extension/ enlargement of an existing cellar or basement or even a swimming pool in your garden.

The requirements of a Basement Impact Assessment can differ significantly from one London Borough to another, with each Local Authority developing their own in-house policy’s, guidance and preferred approaches.  The requirements depend on the borough’s own specific challenges, and this could depend on the number of heritage/listed buildings, population density, closeness to the Thames/ other rivers, geology, topography, flood risk and many other factors.  London’s geography is very much like its people, broad and diverse, so whilst there is no one size fits all approach to basement impact assessment, there is a common approach that most London planning authorities follow.

In general, each Basement Impact Assessment tends to expect the following components to have been extremely well assessed:

• Groundwater – quality, flow obstruction and flooding
• Surface Water – drainage and flooding
• Ground Stability and structural appraisal – slope and impact on existing and neighbouring buildings

Sometimes a Basement Impact Assessment will be called something else, such as a Groundwater Screening Assessment, Basement Method Statement or Structural Methodology Statement.

Sometimes, a desk study and screening stage can be enough to support an application, sometimes you may have to undertake a full impact assessment which could include a site investigation as part of the planning application. This depends on a whole host of factors including the borough your basement is located in. This is discussed more in the next tab.

How is a Basement Impact Assessment carried out?

We have gained valuable experience preparing Basement Impact Assessments for a variety of developments within London. To meet your requirements and those of the Local Authority, we ensure that each Basement Impact Assessment is prepared in accordance with specific planning guidance for that borough, which can be subject to continuous change and updates.

At the very least, a desk-based study is needed to support a planning application. For some sites however, and usually depending on the borough’s specific requirements, an intrusive Site Investigation may also be required to accompany the Basement Impact Assessment and application as well as other specialist reports such as a Flood Risk Assessment, Drainage Strategy, Structural Engineering Plans and Designs, Topographical Surveys and Tree Surveys.

The full scope of the Basement Impact Assessment would be fully understood once the desk study had been completed. Some London Borough’s do accept just a desk-based assessment to support the application, with justification that any further work could be undertaken as a condition of planning, whilst others may insist on a full Basement Impact Assessment to support the application that could take much more time and cost much more to prepare.

A Basement Impact Assessments often comprise five stages, with the first stage being purely desk based and may include a site walkover.

Stage 1: Screening (always required to support an application and supported with a desk-based assessment).

Stage 2: Scoping (we always provide this with the screening assessment if further works are recommended).

Stage 3: Site Investigation, Monitoring and Additional Assessments.

Stage 4: Impact Assessment (may include detailed structural design information, flood risk assessment, drainage strategy, ground movement etc.).

Stage 5: Review and Decision Making (undertaken by local authority).

It is important to know that the requirements of a Basement Impact Assessment can differ significantly from one local planning authority to another, and the stages may vary or the name of the Basement Impact Assessment (i.e. in Westminster they are known as a Structural Methodology Statement).  We work to the requirements of each local council specifically for your site.

If required, the intrusive investigation can also be combined with a geotechnical investigation to inform detailed design. We also work with experienced Structural Engineers during the advanced stages of a Basement Impact Assessment to ensure that this aspect of the assessment is suitably covered.

Frequently asked questions about Basement Impact Assessment

Do I need a Basement Impact Assessment?

If you are building a new, or extending an existing, basement and the site is located within the wider London area you are likely to require a Basement Impact Assessment. Or if you have any planning conditions for your project, the conditions may have details which require a Basement Impact Assessment.

What if I already have a ‘Site Check’ document as part of the sale/purchase of my property?

The danger of a basic site check is that they are usually computer generated. These will usually be rejected by the Local Planning Authority as it means there is the increased potential for oversights, no site walkover (which is more often than not essential), no conceptual model or risk assessment and non-compliant reports. Our Basement Impact are written by qualified consultants who can expertly assess the risks for your site.

How long will it take to prepare a Basement Impact Assessment?

We will usually require 2 to 3 weeks from instruction to complete a desk based study and Stages 1 and 2 and a further 4 to 6 weeks if an intrusive ground investigation is required under Stage 3.  Stage 4 will be 3 to 4 weeks. A full Basement Impact Assessment could take 2 to 3 months to complete. However, if you have deadline to meet, please contact us so that we can consider options to help you.

What is involved in a Stage 1 Screening assessment?

A Stage 1 Screening assessment comprises a desktop study with a review of historical mapping records, geological maps, surface water features, flood maps and regulatory information (including groundwater abstractions etc). We will also conduct a site inspection and walkover (where accessible). The information is then used to inform screening flowcharts, covering three main issues: groundwater (aquifer), land stability, and surface water and flooding.

Can I skip a screening assessment and undertake a Site Investigation instead?

No. The results of the screening flowcharts determines if a full BIA is required, with detailed justification to explain why this is the case. Where it is determined that a full BIA is required and a need to advance, recommendations will be included setting out the next stages of assessment. It is recommended that the completed screening assessment is submitted to the local planning authority for review and comment, ideally before commencing with any Site Investigation work.

How can I be sure I need a Basement Impact Assessment?

Usually, if you are proposing to build a basement or even extent an existing cellar, you would have received pre-application advice on the requirement for a Basement Impact Assessment. You should check with your local planning authority and building control. Otherwise, you may find that you will receive a planning objection when you submit your application.

If you are unsure as to whether you require a Basement Impact assessment, please give us a call, so together we can discuss your requirements. Alternatively, please send an email to info@lustreconsulting.com and provide us with the site location and description of the proposed development.