Free Daylight Assessment Calculator - BRE BR209 45 Degree Rule Screening Tool UK
Analyzing your inputs...
Checking BR209 compliance

Free BRE BR209 Daylight & Sunlight Screening Tool for UK Planning

This free daylight assessment calculator helps homeowners, architects, and planning consultants quickly screen whether a proposed house extension or new building is likely to cause a daylight or sunlight impact on neighbouring properties using the BRE BR209 framework.

Enter your building dimensions and orientation, and the tool instantly calculates VSC (Vertical Sky Component), APSH (Annual Probable Sunlight Hours), and NSL (No-Sky Line) screening values- the same metrics planners and councils assess when reviewing planning applications.

✓ Instant BR209 screening ✓ House extensions & new builds ✓ VSC · APSH · NSL results ✓ 100% free & no sign-up needed

Free Daylight Assessment Calculator (BRE 209 Screening)

BR209 Compliance Check Tool

Check if your extension complies with the 45-Degree Rule • VSC + 25° + ≥3×H • EN 17037 context

VSC
Vertical Sky Component (VSC)

Vertical Sky Component = % of sky visible at a window. A common BR209 screen is 27% (or retaining =0.8× existing).

BRE targets:
• ≥27% = Good daylight
• <27% but >0.8× existing = Acceptable
• <0.8× existing = May need review
Used for: All habitable rooms with windows
 25° / 3×H
25° line & 3×H screen

From the neighbour window, if the line to the top of your proposal is = 25°, that window is often screened from detailed VSC. A conservative check is distance = 3× the height difference.

˜ 25°
Pass if:
• Angle to obstruction =25° (low rise)
OR
• Distance =3× obstruction height
If you pass, detailed VSC often not required!
 APSH
APSH (Sunlight)

Annual Probable Sunlight Hours at a window. South/SE/SW windows receive more; N/NE/NW receive minimal sunlight. Full APSH requires sun-path simulation.

BR209 targets (main living rooms):
• At least 1 window: ≥25% annual + ≥5% winter (21 Sep-21 Mar)
• Reduction to <0.8× existing may need justification
Only assessed for windows within 90° of due south
 NSL / Internal
NSL & Internal Daylight

NSL estimates how much of a room sees sky. EN 17037 & CBDM (UDI/daylight autonomy) give internal targets and quality metrics.

BR209 guidance:
• ≥50% of room area with sky view = Good
• Reduction to <0.8× existing = Review needed
Helps assess internal daylight distribution beyond the window

This estimator shows absolute VSC/NSL/APSH screening values using typical baselines. It is ideal for early risk sense-checks, but a full BR209 report with measured existing data is still required before submission.

Local Sun Data

Get sunrise, sunset & sun angle data

What is Daylight Risk Assessment?

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− Building Geometry

Measure your proposed building that will potentially block your neighbour's light.

Neighbour Your Proposal 6.0 m 2 storeys 3 storeys
Diagram Key
Both buildings shown at their absolute heights.
• Left: Neighbour height
• Right: Your building height
• Dashed line: Distance between buildings
12 storeys15
13 storeys20
0 (party wall)6 m30
3m8 m40m

− Neighbour Window Orientation

Which direction does the neighbour's affected window face?
Orientation Sensitivity
South, SE, and SW facing windows receive the most direct sunlight and are therefore more sensitive to obstruction.
N E S W NE SE SW NW
Click on the compass to select window orientation

− Building Details (helps accuracy)

These details improve calculation accuracy. If unsure, we'll use typical values.

Narrow
Single house (~10m)
Typical
Semi/terrace (~20m)
Wide
Long block (~50m+)
Ground
Floor 0
First
Floor 1
Second
Floor 2
Third+
Floor 3+
Small
1.0m × 1.2m
Medium
1.5m × 1.5m
Large
2.0m × 2.0m
Patio Door
2.0m × 2.4m
2.5m4.5 m8m

− Room & immediate context

Select the room type and context that best matches.

Living / Kitchen
Most sensitive
Bedroom
Moderate
Other room
Less sensitive
Open
Wide gaps / setbacks
Partial
Typical street width
Courtyard
Narrow / canyon-like
Balcony / overhang
Soffits above windows
Dense urban
Generally low daylight
Conservation/listed
Stricter scrutiny

Pro inputs (optional)

Refines NSL/internal and garden overshadow notes. We’ll sanity-check values.

− Get your results

Enter your details to reveal your tailored results + a neat PDF snapshot.

By submitting this form, you give consent for Environmental Economics to store and process your information to provide your results and follow-up support. We may contact you occasionally about related services, and you can withdraw consent or request deletion at any time. Full details are available in our Privacy Policy.

What Happens Next?

This calculator gives you an instant BRE BR209 screening result - but a screening alone is not sufficient for a formal planning application. If your result shows Medium or High risk, or your council has specifically requested a daylight and sunlight assessment, the next step is commissioning a full BR209 Daylight & Sunlight Report from a qualified consultant.

1 Get in touch - describe your project and we will confirm whether a full report is needed.
2 We survey & model - existing daylight at neighbouring windows is measured and your proposal accurately modelled to BRE BR209.
3 Report delivered - typically within 5–7 working days, accepted by all UK planning authorities.

Ready to get a full BR209 Daylight & Sunlight Assessment? We respond within one working day.

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