This will usually depend on the size, height and location of the proposed new building.
If you are positioning the building close to a boundary or in front of the building line of your home when viewed from the road, then planning consent is usually required.
If the building is to be behind the house, more than 2.0m from a boundary and provided the ridge height is less than 4.0m, then you may not need permission, assuming it is for non business use. Most garden sheds and chalets therefore do not need local authority permission.
However, if you live in a listed property or within a conservation area then planning permission is nearly always required.
The Government Planning Portal www.planningportal.gov.uk contains a wealth of additional information and includes an interactive guide covering most circumstances.