Hounslow publishes local Building Safety Levy (BSL) rates, but a rate alone does not decide whether a development is liable. The England-wide charging tests, the development’s planning context, the person responsible for the levy, and the rules in force for the application must be considered together.
Check the national charging tests
The national guidance says the levy applies only where all relevant charging conditions are met: the project is a major residential development, creates new chargeable residential floorspace, and the client is not exempt from paying. The guidance defines major residential development around schemes of 10 or more dwellings or purpose-built student accommodation with 30 or more bedspaces.
A single standalone dwelling or ADU would ordinarily be below the stated unit threshold. However, the guidance says a building-control application can relate to a larger development covered by a major planning permission. Do not assess the threshold from the ADU application alone if it forms part of a wider scheme. The collecting authority must confirm how the permission and chargeable floorspace tests apply.
Read Hounslow’s rate and effective date carefully
Hounslow’s published page lists a rate of £58.29 per square metre and a reduced rate of £29.15 per square metre for qualifying previously developed land. The Council says these rates apply to applications submitted on or after October 1, 2026. That effective date has passed. Check the current Council page for any later rate or rule change before making an application.
The lower previously-developed-land rate is conditional. Confirm the Council’s definition and evidence requirements for the site, the area of chargeable floorspace, the responsible client, and any exemption or relief before estimating an amount. A published rate is not a parcel calculation.
Ask the authority to confirm the project position
Before including the levy in a project budget, ask the relevant local authority whether the proposal is part of a major development, what floorspace is chargeable, which rate and application date apply, whether the land qualifies for the reduced rate, and who is liable. Keep the levy distinct from Community Infrastructure Levy, building-permit fees, utility charges, and construction costs.
For other England charge examples and local charge types, see the development and utility charge guide. That guide also explains why an ADU label alone does not establish that a charge applies.