Rights to Light Calculations

A common myth is that rights to light can be assessed using the ‘45 degree rule’. The 45 degree rule is
often used to assess planning applications but is not used in legal rights to light cases.

The so called ‘50:50 rule’ is generally accepted as the appropriate way to measure light levels for rights
to light cases. The 50:50 rule involves calculating the percentage of a room’s area which can receive
adequate light. The calculations are undertaken at a working plane 850mm above the floor. A point on the
working plane is considered adequately lit if it can receive at least 0.2% of the total illumination received
from the sky. An injury is generally deemed to be caused where the area of a room receiving light from at
least 0.2% of the sky is reduced to less than 50% to 55%.

The calculations were historically undertaken using ‘Waldram Diagrams’. Nowadays specialist computer
software programmes are used to undertake the calculations.

Define maximum building envelopes
Risk analysis reports
Dispute resolution
Expert witness reports
Light obstruction notices