Free Daylight Assessment Calculator — BRE 209 Screening Tool
Analyzing your inputs...
Checking BR209 compliance

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.

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