There is no single legal definition of "habitable room", as its use and meaning are subject to context. Generally, a habitable room is a space in a building that is designed for people to live, sleep, or spend extended periods of time in, but does not include bath or toilet facilities, corridors or utility rooms.
To qualify as permitted development some household building work projects – notably extending a house – must not exceed limits to the volume being added.
In England, the volume allowance mostly applies to loft conversions. You can read more on the Planning Portal.
Use the volume calculator to find out the volume of several loft conversion and extension types.
Yes, but you will need to obtain planning permission. Converting a detached garage into a self-contained living space, such as annexe, studio flat, or retail unit, represents a material change of use, particularly if it will function independently from the main house. This is because the garage would no longer be used in connection with the main dwelling and would effectively become a separate residential unit.
For houses, replacing windows with double glazing usually doesn’t require planning permission, provided the style and appearance are similar to the original. However, if you live in a flat/maisonette, listed building, or a conservation area, planning permission or listed building consent may be required, especially if the changes affect the external appearance.
Read more about planning permission for windows and doors for houses
Read about planning permission for windows and doors for flats and maisonettes
The conversion of a garage, or part of a garage, into habitable space, will normally require approval under the building regulations.
As part of the garage conversion, it is likely that the original garage door will be in-filled with a new wall and possibly a window or door, and a new foundation may be needed for the new wall.
As well as the foundation, you also need to consider:
Read more about the building regulations for garage conversions
Permitted development rights allow certain types of work to be carried out without the need to apply for planning permission. It typically covers things like small extensions, loft conversions and outbuildings, provided they meet specific size and location limits. These rights can vary or be removed in areas like conservation zones and by Article 4’s, so it’s best to check before proceeding.
Read about permitted development and Article 4 directions on Planning Portal