Emergency Egress Windows
Here is some information on how to meet UK Building Regulations, Approved Document B, clause 2.8.
Any window provided for emergency egress purposes should have an unobstructed openable area that is at least 0.33m2 and at least 450mm high and 450mm wide.
The bottom of the openable window should not be more than 1100mm above the floor.
Please see the Approved Document for more information.