What size home extension can I build without planning permission?

As long as your extension qualifies under the prerequisites of permitted development rights then you can extend without permission. As an example, size restrictions and the keeping within of the existing finish of the property will apply for the addition of a single storey extension.

The specific conditions of permitted development will not apply if your property is a listed building. There is also a good chance that if you own a property in an area of natural beauty or conservation area that permitted development will not be applicable either.