A new ‘permitted development’ right for the upward extension of houses is being introduced from 31 August 2020 (known as Class AA - enlargement of a dwellinghouse by construction of additional storeys).
It enables up to 2 additional storeys to be added to existing houses (detached or terrace) to create an extension to the existing house, or to create a new home (i.e. a flat above the existing house). The overall height of the building, once extended, must not exceed 18m.
A single storey house (bungalow) can add one additional storey.
This new permitted development is subject to the Prior Approval of the Local Planning Authority on matters including external appearance and the potential impact of the extension on the amenity of neighbours (including overlooking and loss of daylight/sunlight). This means that you have to apply to your Local Planning Authority for their agreement that the upwards extension complies with all the criteria set out in the regulations– you’ll need to satisfy them that the design is appropriate and that there is no undue impact on other nearby residential properties. If prior approval is given, then you will also need to provide details of how you intend to manage construction on the site, and you must complete the development within 3 years of approval. As of December 2020, prior approval applications where the height of the existing building is 18m or more also require a report confirming that the external wall construction of the existing building meets building regulations requirements for resisting fire spread.
There are many caveats where the PDR does not apply and full planning permission will therefore be required – this includes properties in Conservation Areas, sites of Special Scientific Interest, on houses constructed before 1st July 1948 or after 28th October 2018, on properties where there is already an extension above the original top floor.
The detailed Regulations can be found here: https://www.legislation.gov.uk/uksi/2020/755/made