Design & Planning: What is a Basement Impact Assessment?

Across London high land values have given rise to an increasing level of below ground development. With careful design digging down makes sense; basements can be light filled with generous ceiling heights adding much needed space and value to your home. However, getting planning permission can be tricky, often with nervous neighbours scrutinising your every move. Following in the footsteps of central London boroughs, Lambeth Council is soon to adopt specific basement planning guidance. This article summarises what you need to prepare for when considering a basement development.

Hackney Extension - underpinning, sequenced construction, specialist tanking and drainage basement systems

In most circumstances a basement will require planing permission. Lambeth’s Draft Basement SPD stipulates what constitutes a basement development: “Basement development includes any excavation to form new or additional floor space below ground level of an existing or new development; or work which requires underpinning.” One of the most common basement projects is an extension to the coal cellar under the stairs of a Victorian House. Under the new SPD guidance this would require a Basement Impact Assessment.

Basement Design Guidance:

Basements can be light-filled, delightful spaces with high ceilings, high quality design is imperative and this is reflected in what the council will ask you to consider. (These are specific to Lambeth however all London boroughs have corresponding design guidance – see below for links to other London Boroughs).

  • Size – Basement footprint should be ‘subordinate’ to the original dwelling and occupy less than 50% of the original garden.
  • Basement depth – generally preferred to be one storey only to a maximum ceiling height of 3m.
  • Habitable rooms – bedrooms and living spaces must have suitable access, space, daylight, outlook.
  • Flood risk – habitable rooms will be resisted in high risk flood zones (3a & 3b).
  • External appearance above ground must be high quality and respond positively to the host building.
  • Lightwells should be minimal, close to the original host building, be ‘in-keeping’ and not reduce parking provision.
  • Steps to the front of the building should be avoided if not a characteristic of the street.
  • Landscape integrity of the front gardens should be retained and enhanced, extensions below front gardens preventing planting will be resisted, and trees of value should be retained.
  • Exposed below ground elevations are to be subordinate and in-keeping with host building in terms of size, window treatment, materiality and detailing.
  • Basements to Heritage Assets (buildings in Conservation Areas or Listed Buildings) will be further scrutinised.

Requirements – how many surveys and reports are necessary?

Due to the complexity of basement construction clients are often surprised at the amount of supporting consultation required. On the other hand, if you are a neighbour, the required professional input offers some reassurance that the project will be carried out successfully. Here is a summary of what Lambeth may ask for:

  • Trees – An Arboricultural Impact Assessment (AIA) may be required to assess the impact of the development on trees of value.
  • Flood Risk – A Flood Risk Assessment may be required to show what, if any, flood mitigation measures are required.
  • Managing Construction Impacts to neighbours – A draft Construction Management Plan (CMP) may be required outlining how the basement contractor intends to construct the basement. This will then get finalised and re-submitted pre-commencement.
  • Basement Impact Assessment (BIA) – An assessment of the flood risk, drainage and land stability at the site, what the impact of the development will be, and what mitigation measures may be required. There is some duplication with the CMP however the BIA is principally a process of investigation and assessment and will include:
    • Site analysis of hydrological and geotechnical conditions.
    • Proposals for appropriate waterproofing, tanking and drainage.
    • Proposed measures to mitigate risks against land stability.
    • Details of basement construction methods.
    • Details of construction programme and site set-up.

As you can see there are multiple consultants required to complete the necessary planning submission reports. Site investigations, structural engineers, specialist ground and hydrology engineers will need to be consulted as a minimum. Undoubtably, in some instances, this can appear like taking a sledgehammer to crack a nut, particularly if your basement is on the small side!

Technical Detail - Section through sequenced underpinning with Delta Drain MS-20 & MS-500 tanking system

Process – Focus on Design

Should you wish to carry out a basement extension our first piece of advice is to remain focused on the quality of the design. Whilst it is imperative that the technical aspects of planning and construction are carried out professionally, the finished product is what will create delight in your everyday life. So first of all find an experienced architect whose work resonates with you and ask them to assess the viability of your scheme. They will be able to guide you through the design process. Its a good idea to start the site investigations as soon as possible, we would also recommend getting a structural engineer on board early. The technical aspects of the Basement Impact Assessment will typically be outsourced to specialist companies. Once planning is in place you should continue with a full professional service from your architect and engineer in order to ensure that the technical aspects are designed and subsequently built correctly. This will require professional supervision on-site from your design team.

Lambeth vs London

At the time of writing Lambeth’s Basement SPD has not yet been formally adopted. However we have noticed that Basement Impact Assessments are being requested in spite of this. BIA’s were developed by Kensington and Chelsea and are now standard in a lot of London boroughs – Westminster, Camden, Islington, Haringey and Southwark  included. Wandsworth also has specific basement guidance, (for further reading you can click on the links above).

Should you have a project that you are considering please don’t hesitate to get in touch. For contact details click here, and you can view a selection of projects here.