Planning Permission - Permitted Development rights
‘permitted development’ rights exist that allow homeowners to extend a house without needing to apply for planning permission. The Government website details what is classified as “permitted development”
If you want to exceed these, then it is likely that an application for householder planning permission will be required.
For all extensions:
Only half the area of land around the "original house" can be covered by extensions or other buildings.
Extensions cannot be higher than the highest part of the existing roof; or higher at the eaves than the existing eaves.
Where the extension comes within two metres of the boundary the height at the eaves cannot exceed three metres.
Extension cannot be built forward of the ‘principal elevation’ or, where it fronts a highway, the ‘side elevation’.
For Side Extensions:
Where it would extend beyond the ‘side elevation’ of the original house, the extension:
Cannot exceed four metres in height.
Can only be a single storey.
Can only be up-to half the width of the original house
Single storey extensions:
Single-storey rear extensions cannot extend beyond the rear wall of the original house by more than four metres if a detached house; or more than three metres for any other house
Single-storey rear extensions cannot exceed four metres in height
Extensions of more than one storey:
Extensions of more than one storey must not extend beyond the rear wall of the original house by more than three metres or be within seven metres of any boundary opposite the rear wall of the house.
Roof pitch must match existing house as far as practicable (note that this also applies to any upper storey built on an existing extension).
Any upper-floor window located in a ‘side elevation’ must be obscure-glazed; and non-opening (unless the openable part is more than 1.7 metres above the floor).
All side extensions of more than one storey will require householder planning permission.
More details can be found on the planning portal website.
Clearly it is complicated, your architect is there to help you!