Start with the policy requirement
The statement should identify the applicable route and explain the proposed amount, type and delivery of affordable housing. It must use the correct measurement basis and be consistent with the accommodation schedule and any proposed planning obligation. Westminster's validation list identifies affordable housing statements and viability assessments as distinct documents. [1]
Do not commission a viability report simply because a project has costs. Policy DM5 of the NPPF published on 17 August 2026 says that policy-compliant proposals should be assumed viable and identifies circumstances that may justify an application-stage assessment. These include materially different development or site characteristics, unaccounted-for costs, and significant changes in relevant circumstances. [2]
What does viability evidence examine?
The assessment brings together development value, costs, land value and the other relevant assumptions. National guidance requires a standardised, evidenced approach and explains benchmark land value. The price paid for the site is not, by itself, justification for failing to meet policy. [3] [2]
Ask the assessor to explain the evidence behind the inputs and the reasons for any departure from the assumptions underlying the plan. A report should not merely present a preferred financial outcome with unexplained numbers. National policy also expects viability assessments to be publicly available. [2]
What changed in London in March 2026?
The March 2026 housing-support guidance introduced a temporary route for eligible residential schemes. For qualifying private-land proposals, it uses a 20% affordable housing threshold measured by habitable rooms. The guidance sets different requirements for public and relevant industrial land, together with tenure, eligibility and review conditions. It is not a universal 20% rule for every London site. [4]
Important exclusions include Green Belt and grey belt proposals, specified student or shared-living schemes, and proposals involving demolition of existing affordable housing. Eligible schemes following the full temporary route do not need an upfront viability assessment, but must satisfy its delivery and review arrangements. Applications must be validated by 31 March 2028 to use that route. [4]
Check the whole eligibility test before preparing the statement. A headline percentage alone does not establish which route applies to the development.
Who prepares the documents?
A planning consultant can coordinate the affordable housing statement with the designer and relevant housing advice. A viability assessment needs appropriate valuation and development-cost expertise. National guidance expects evidence supporting the financial inputs, not an unsupported calculation by the applicant. [3]
Clarify whether the appointment includes responding to the council's review and updating the assessment after design or cost changes. Keep the unit schedule, tenure assumptions and financial model aligned.
Check the position at submission
The temporary guidance is current guidance with a defined eligibility window; the draft new London Plan is a separate, unadopted proposal. Identify which route the application relies on and recheck any changes before submission. [4] [5]