Do you need planning permission to replace windows?
When planning permission is not required
Most houses have permitted development rights, which make it possible to replace existing windows (or create new windows) without planning permission.
The materials of the proposed windows must be 'of a similar appearance' to the materials of the existing windows, and any upper-floor side windows must be obscure-glazed and non-opening. This does not apply if the parts of the window that can be opened would be more than 1.7 metres above the internal floor level.
When planning permission is required
Not all houses have permitted development rights. Sometimes these rights have been removed by an Article 4 direction because the building is within a conservation area or because a condition was attached to a previous planning permission which removed the rights. If permitted development rights have been removed then you will need planning permission.
Flats and maisonettes
Flats and maisonettes do not have permitted development rights to replace or insert windows, so planning permission is required.