Competence for a retail impact assessment
Retail impact assessments are tested by the authority's own consultant, input by input. The skills needed, where the survey data comes from and which assumptions get attacked.
Nobody is licensed to write a retail impact assessment, so if you are asking who writes a retail impact assessment the real answer is: whoever the authority's own retail consultant will not take apart. That is the standard to appoint against, because on any contested out of centre scheme the council will commission a review and the argument will be conducted between two specialists over survey data and growth assumptions.
The threshold that brings you into this is the one in national policy. Proposals for retail, leisure and office development outside town centres, which are not in accordance with an up-to-date development plan, need an impact assessment where the floorspace exceeds a proportionate locally set threshold, or 2,500 square metres of gross floorspace where the plan sets none.
What the assessment has to establish
Two tests, both drawn from the National Planning Policy Framework (August 2026 edition), and they are often conflated.
The first is the impact on existing, committed and planned public and private investment in a centre or centres within the catchment area. That is about whether your scheme undermines a town centre scheme that has permission, funding or an allocation.
The second is the impact on town centre vitality and viability, including local consumer choice and trade in the centre and the wider retail catchment. The assessment period runs to five years, extended to ten years for major schemes whose full effect will not be felt within five.
Where a proposal fails the sequential test or is likely to cause the kind of adverse impact national policy treats as serious on either limb, national policy says it should be refused. That makes the arithmetic consequential rather than presentational.
The sequential test is a different exercise with a different method, and the two documents should not be merged. A Sequential Test (Retail) asks whether sites in or edge of centre are available, suitable and viable. The impact assessment assumes you have passed that and asks what your scheme does to the centres.
Retail planning and economics skill sets
The work needs two capabilities and they rarely sit in one head.
Retail planning, meaning a chartered planner who understands the policy tests, the case law on sequential and impact, and how inspectors have handled trade diversion arguments at inquiry.
Retail economics, meaning someone who can build and defend a quantitative model: catchment zones, population projections, expenditure per head, special forms of trading deductions, floorspace efficiency growth, sales densities and turnover estimates.
A good team adds a third element: data handling. Catchment modelling and survey analysis are statistical tasks, and an assessment with a spreadsheet nobody can audit is an assessment that falls over under scrutiny.
Ask three things of any prospective author. Which inquiries or hearings have you given evidence at on impact. Show me a model I can open and follow. And which authority's consultant have you been cross-examined by.
Household survey data and its sources
The survey is the foundation and it is the most expensive line.
A household survey, conducted by telephone or online across defined catchment zones, establishes where residents currently shop for convenience and comparison goods, which produces the market shares that drive everything downstream. Sample sizes per zone, the questionnaire wording and the zone boundaries are all contestable, which is why they should be agreed before the survey runs.
Three practical points.
- An existing survey commissioned for the authority's own retail study can often be used, which saves a great deal of money and removes an argument about methodology.
- Survey data ages. A study based on a survey four or five years old will be challenged, particularly where shopping patterns have shifted.
- Population and expenditure projections come from commercially published datasets, and the growth rates applied to comparison goods expenditure are among the most heavily disputed inputs in the whole exercise.
Office for National Statistics population and census data provides the demographic spine, and a model that departs from it needs to say why.
Trade diversion modelling and its assumptions
Trade diversion is the heart of the document. The model estimates your scheme's turnover, then allocates the source of that turnover across the centres and stores in the catchment, then expresses the loss to each centre as a percentage of its turnover.
| Input | Why it is argued about |
|---|---|
| Sales density | Turnover per square metre assumed for the new floorspace |
| Catchment zones | Zone boundaries determine market shares and diversion patterns |
| Special forms of trading | The deduction for online and non-store retailing |
| Expenditure growth | Growth rates applied over the assessment period |
| Floorspace efficiency | The assumed improvement in existing traders' sales densities |
| Diversion pattern | How much trade comes from which centre, as opposed to new expenditure |
The honest difficulty is that there is no single accepted method for the last of these. Diversion is estimated using survey-derived market shares, distance and the character of competing provision, and two competent consultants can produce materially different allocations from the same survey.
So the assessment has to be transparent. State each assumption, give the sensitivity, and show what happens to the headline impact if the contested inputs move. A model that only produces one answer invites the authority's consultant to produce a different one.
Town centre health check evidence
The quantitative work needs a qualitative counterpart, because an impact of a given percentage matters more in a fragile centre than a strong one.
A health check assembles indicators for each affected centre: the diversity of uses by type and floorspace, the proportion of vacant units at street level and how long they have been vacant, retailer representation and evidence of demand, rents and yields where available, pedestrian flows, accessibility and the physical quality of the environment.
Collect it on foot. A vacancy survey from aerial imagery or a desktop database will miss recently closed units and temporary occupiers, and a case officer who walks the high street weekly will know.
Where the scheme also claims economic benefits, the employment and expenditure case belongs in an Economic Impact Assessment rather than being folded into the retail work, because the methods and the audiences differ.
Where the authority's own consultant will test it
Expect a review and expect it to focus on five things: the survey base and its age, the sales density assumed for your floorspace, the special forms of trading deduction, the expenditure growth rates, and the diversion pattern.
The review will also test whether your conclusion follows from your own numbers. An assessment that reports a 12 per cent diversion from a small town centre and concludes there is no material adverse impact needs to explain why, by reference to that centre's resilience, rather than asserting it.
When the two specialists disagree, the practical question becomes how the authority resolves it, and the dynamics of that are covered in when the council's expert disagrees with your consultant. The short version is that narrowing the dispute to two or three inputs, in writing, is far more productive than defending everything.
A Retail Impact Assessment is labelled as professionally recommended rather than legally required, which means there is no statutory barrier to drafting one in-house. Councils expect a suitably qualified author, and on a contested scheme professional review before submission is worth more than professional defence afterwards.
Scoping the study area first
The sequence that saves money runs in this order.
- Ask the authority which threshold applies, local or the national 2,500 square metres figure.
- Ask whether it has a current retail study and whether its survey can be used.
- Agree the catchment zones, the base year and the assessment period in writing.
- Agree which centres are to be assessed, and which are agreed to be unaffected.
- Agree the treatment of special forms of trading and the expenditure growth source.
- Only then commission the survey or the modelling.
Getting items three to five agreed converts a contested methodology into a contested conclusion, which is a much better position. An authority that has signed off the inputs cannot credibly instruct a consultant to attack them.
Browse all 66 report types on PlanScope to see where the retail, sequential and economic documents sit in an out of centre application.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
When is a retail impact assessment required?
Where retail, leisure or office development is proposed outside a town centre, is not in accordance with an up-to-date development plan, and exceeds a proportionate locally set threshold. Where the plan sets no threshold, national policy applies a default of 2,500 square metres of gross floorspace. Ask the authority which figure applies before scoping.
Who is qualified to prepare one?
There is no licence or statutory competence standard. In practice the work needs a chartered planner who knows the policy tests and appeal practice, a retail economist who can build and defend the quantitative model, and someone who can handle the survey and population data. Ask any prospective author which inquiries they have given impact evidence at.
What are the two impact tests?
The effect on existing, committed and planned investment in centres within the catchment, and the effect on town centre vitality and viability including consumer choice and trade. The assessment period is five years, extended to ten for major schemes whose full effect arrives later. A likely serious adverse impact on either limb should lead to refusal.
Where does the household survey data come from?
A telephone or online survey across defined catchment zones, establishing where residents shop for convenience and comparison goods. Where the authority has commissioned a recent survey for its own retail study, using it saves money and removes an argument about methodology. Survey data more than a few years old will be challenged.
Which assumptions get attacked most?
The age of the survey base, the sales density assumed for the new floorspace, the deduction for special forms of trading such as online retailing, the expenditure growth rates applied over the period, and the trade diversion pattern between centres. Present sensitivities for each rather than a single answer.
How does it differ from the sequential test?
The sequential test asks whether sites in or on the edge of a centre are available, suitable and viable for the proposal. The impact assessment assumes that test has been passed and quantifies what the out of centre scheme would do to existing centres. They use different methods and should remain separate documents.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.