Re-running the daylight model for a section 73 change
Screen the amendment against four geometric parameters before commissioning anything. Where reassessment is needed, the approved scheme is the comparator that answers the planning question.
A section 73 daylight reassessment is needed less often than officers ask for it and more often than applicants offer it. The deciding question is narrow: does the amended massing change the geometry seen from any window or amenity area that was assessed on the original application. If it does not, a one page comparative note with a drawing overlay is the proportionate answer. If it does, the modelling has to be re-run, and the comparator has to be chosen with care, because the approved scheme and the existing situation give different answers and both have a role.
Section 73 of the Town and Country Planning Act 1990 allows permission to be granted for development without complying with a condition previously imposed. The authority considers only the condition in question, and the original permission stays alive whatever happens.
When a change triggers reassessment
Three categories of amendment reach daylight and sunlight.
Changes to the envelope towards a sensitive boundary: an increase in height, a deeper footprint, a parapet raised for plant, a storey height increase that lifts the whole building, or an infill of a previously open recess.
Changes to openings in the scheme's own facades, where the amendment affects daylight to the new units rather than to neighbours. Reducing a window, adding a balcony above it, or deepening a reveal all do this, and internal assessments are easier to forget than neighbour assessments.
Changes to external amenity space, including a new terrace, a relocated play area or a courtyard losing a side. The sun on ground test is sensitive to small moves in massing because it depends on shadow paths rather than on sky angles.
Everything else, including material changes, internal layout within an unchanged envelope, and fenestration changes on elevations facing nothing, needs no reassessment. Say so with reasons rather than silently omitting the topic.
Comparing against the approved scheme rather than existing
This is where submissions go wrong. The numerical tests in the BRE guidance are framed against the existing situation, so a formal vertical sky component or no sky line ratio is calculated against what is there now. That is the right baseline for the published criteria and it should be reported.
But the planning judgement on a section 73 application is about the effect of the amendment. The useful comparison for an officer is approved against proposed, because the approved scheme is the fallback: it can be built, and a neighbour's position already reflects it. A table with three columns, existing, approved and proposed, answers both questions at once and takes no more work than a table with two.
Where the amendment improves matters against the approved scheme while both fail against existing, say that plainly. It is a common and perfectly respectable position, and an assessment that reports only the failure against existing invites a refusal on a point the amendment did not cause.
| Comparison | What it tells the officer | Where it belongs |
|---|---|---|
| Existing against proposed | Compliance with the published numerical criteria | The results tables |
| Approved against proposed | The effect attributable to the amendment | The conclusions and the covering note |
| Approved against proposed, amenity areas | Whether shadow paths have worsened | The sun on ground plots |
Parameters that move results most
Window head height on the receiving building, which drives the no sky line more than anything else and is a property of the neighbour rather than of your scheme.
Distance from the amended face to the receiving window, where small reductions matter disproportionately at close range.
Height of the amended element above the horizontal from the receiving window. An increase in parapet height on a building 8 metres away changes the vertical sky component materially. The same increase 40 metres away does almost nothing.
Projection depth. Adding or deepening a wing, a balcony stack or a plant enclosure on the flank facing a neighbour cuts obliquely into the sky view and often has more effect than a general height increase.
Screen the amendment against those four before commissioning anything. A change that moves none of them does not need a model run.
Presenting a short comparative note
For an amendment that touches the envelope lightly, four elements do the job.
- A drawing overlay showing approved and proposed massing, with the changed element highlighted and dimensioned.
- A statement of which assessed windows and amenity areas lie within the affected field of view, and which do not, with the reasoning.
- Results for the affected receptors only, in a three column table.
- A conclusion in two sentences on the effect attributable to the amendment.
Signed by the same consultant who did the original assessment wherever possible. Continuity of authorship removes the question of whether the model inputs are the same, which is otherwise the first thing a reviewer asks. Councils generally expect a suitably qualified author for this work and professional review is strongly advised.
Where the original report assessed a neighbour's room served by more than one window, make sure the amended assessment treats it the same way, because that is a modelling choice rather than a fact. Our note on when a neighbour's room is served by two windows explains why the choice changes the result.
Where a full reassessment is unavoidable
Four situations call for the complete exercise rather than a note.
An increase in storeys or in overall height on an elevation facing residential windows within roughly 25 metres. A change to the building footprint that moves any face closer to a boundary. The addition of a substantial new element such as a plant enclosure, a lift overrun or a roof extension. And any amendment to a scheme where daylight was contentious at the original application, regardless of scale, because the planning history makes it a live issue and the committee will expect numbers.
Add a fifth in practice: where the original model is no longer available. If the original consultant has gone, the model file is lost, or the software has moved on, rebuilding from the approved drawings is a full exercise whatever the amendment.
Conditions that require it
Read the original decision notice before deciding anything. Conditions commonly require development to be carried out in accordance with an approved daylight and sunlight report, which makes any departure a breach rather than a judgement, and the section 73 application then has to vary that condition as well as the drawings condition.
Other wordings require a revised assessment to be submitted and approved before works above a stated level, or tie mitigation such as obscure glazing or a privacy screen to the assessment. Where mitigation was conditioned, check that the amendment has not removed the element the mitigation was attached to.
Note the drafting point on scope: a section 73 application cannot change the description of development, and the Levelling-up and Regeneration Act 2023 introduced a separate route at section 73B for amendments, with commencement arrangements to be verified against the current position. Confirm which route is open before you choose one.
Avoiding an unnecessary exercise
Screen first, in writing, and send the screening to the case officer before you commission anything. Half a page: what is changing, which receptors could be affected by geometry, why the rest cannot be, and what you propose to submit.
Officers accept that approach far more often than consultants expect, because it gives them something to quote in the report. The alternative, submitting with no daylight content at all and waiting to be asked, costs a month.
Where the amendment affects amenity space rather than windows, an Overshadowing Study limited to the affected area is usually quicker and clearer than reissuing a full Daylight and Sunlight Assessment. Scope it to the question asked.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. To see which supporting documents an amendment application is likely to need, enter your postcode on PlanScope.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Does every section 73 application need a new daylight assessment?
No. Reassessment is needed where the amendment changes the geometry seen from a window or amenity area that was assessed originally. Material changes, internal layout alterations within an unchanged envelope and fenestration changes on elevations facing nothing sensitive do not need it. Say so with reasons rather than omitting the topic silently.
Should I compare against the existing situation or the approved scheme?
Both. The published numerical criteria are framed against the existing situation, so those ratios belong in the results tables. The planning judgement on an amendment concerns the effect attributable to the change, which means comparing approved against proposed. A three column table covers both without extra modelling.
Which parameters matter most when screening an amendment?
The receiving window's head height, the distance from the amended face to that window, the height of the amended element above the horizontal from it, and the depth of any new projection. An amendment that moves none of those four does not need a model run, and the screening note should explain why.
What if the original daylight model no longer exists?
Treat it as a full reassessment. Rebuilding the model from approved drawings takes the same effort whatever the amendment, and the rebuilt baseline should be checked against the original report's published figures so that any difference in inputs is identified and explained rather than discovered by a reviewer.
Can a condition make reassessment compulsory?
Yes. Conditions often require development to be carried out in accordance with an approved daylight and sunlight report, which turns any departure into a breach. Others require a revised assessment before works above a stated level, or tie mitigation to the assessment. Read the decision notice before deciding what the application has to vary.
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.