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!
