Adding a new window to an existing house wall is a relatively minor alteration that is permitted development in most circumstances. However, there are specific situations where planning permission is required, particularly where the new window could create overlooking issues or affect the character of a protected area or building.

Adding Windows: The General Position

For most dwellinghouses in England, adding a new window to an existing wall is permitted development under the general alterations class of Part 1 of the General Permitted Development Order. Provided the window does not result in a material increase in the size or footprint of the building, and provided the specific conditions below do not apply, a planning application is not required.

Side Elevation Windows: Obscure Glazing Requirement

Adding a window to a side elevation of a house, meaning a wall facing toward a neighbouring property rather than toward the rear or front garden, is permitted development only if the window is on the ground floor, or if it is at first floor level or above and is obscure-glazed and non-opening in the part below 1.7 metres from the floor. This condition prevents overlooking of neighbouring properties from first-floor or higher side windows. A clear-glazed openable side window at first floor level requires planning permission unless the building is in a position where the overlooking issue does not arise.

Conservation Areas

In conservation areas, cladding the exterior and certain alterations to windows require planning permission. Adding a new window opening to the front or side elevation of a house in a conservation area that would materially affect the appearance of the building may require planning permission, particularly if it would be inconsistent with the character of the original building or the area.

Listed Buildings

Adding any window to a listed building requires listed building consent, and in most cases planning permission as well. Even a small new window opening in an unlisted location within the building affects the historic fabric and is subject to consent requirements. The local planning authority's conservation officer will consider whether the proposed window is appropriate to the character of the building.

Flats and Maisonettes

The residential outbuilding and alteration permitted development rights do not apply to flats and maisonettes in the same way as to houses. Alterations to flats that affect the external appearance generally require planning permission. Adding a window to a flat is likely to require planning permission unless it is a straightforward like-for-like replacement.

Summary

Adding a new window to a dwellinghouse is generally permitted development in England. First-floor and above side elevation windows must be obscure-glazed and non-opening below 1.7 metres. Conservation areas and listed buildings impose additional requirements. All new window installations must comply with Building Regulations thermal standards, either through a FENSA-registered installer's self-certification or through Building Control. Flats are generally subject to more restrictive rules.