The planning regime for flats and maisonettes differs in many important ways to that which covers houses.
You may need to apply for planning permission to fit new windows (such as double-glazing) in your flat or maisonette. You will not need planning permission to add internal secondary glazing.
You should not need planning permission if you are fitting windows which are identical, like-with-like replacements. However, if the new windows differ in appearance or size to those you are replacing (for instance, different glazing patterns) you may need to apply for planning permission.
Local policy and interpretation of the rules covering changing windows in flats varies from council to council and you are advised to contact your Local Planning Authority for advice before starting work.
If you are a leaseholder, you may first need to get permission from your landlord, freeholder or management company
If your flat is in a listed building it is very likely you will need listed building consent and you should contact your local planning authority for advice on design and materials before you start work. Works to a listed building that affect its special historic character without consent is a criminal offence.
If your flat is in a conservation area and your proposals involve an element of demolition then you may need to apply for conservation area consent and you should contact your local planning authority for advice.
Since 1 April 2002 building regulations have applied to all replacement glazing. The regulations apply to thermal performance and other areas such as safety, air supply, means of escape and ventilation.
Disclaimer - Wales
This is an introductory guide and is not a definitive source of legal information. This guidance relates to the planning regime for Wales. Policy in England may differ. If in doubt contact your Local Planning Authority.