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.
Yes, under permitted development, a new dormer must not be higher than the existing highest part of the roof. It should also have materials that are in similar appearance to the existing roof. Additional limits apply in conservation areas and on listed buildings. Read about planning permission for loft conversions on Planning Portal
A wall is a 'party wall' if it stands astride the boundary of land belonging to two (or more) different owners, for example the wall separating two semi-detached properties.
Some kinds of work carried out to a property may not be controlled by the building regulations, but may be work which is covered by the Party Wall etc. Act 1996. This is a separate piece of legislation with different requirements to the building regulations.
To find out more about party walls, visit Planning Portal.
If you're doing electrical installation work in your home or garden in England or Wales, you must follow the building regulations. It's recommended to hire an installer who is registered with a competent person scheme (a registered competent person’) who can self-certify that the work meets the required standards.
In many circumstances, rules known as ‘permitted development’ rights allow you to extend a house without needing to apply for planning permission, providing specific limitations and conditions are met. If you want to exceed these limits and conditions, then it is likely that an application for householder planning permission will be required.
You can find information about the limits and conditions, prior approval and the building regulations on the Planning Portal by using the links below.
Read about planning permission for extensions
Read about prior approval for larger single storey extensions
Find out about building regulations for an extension
If the garage will remain in domestic use - for example, as a personal hobby or home workshop - then planning permission is typically not required.
However, it’s important to check whether there are any planning conditions, covenants, or restrictions attached to your property or the garage itself that may limit or prevent conversion.
Read about planning permission for garage conversions on Planning Portal.