Adding an Extra Storey: The New Permitted Development Rules

The right to add upward extensions is granted under Class AA of Part 1 of the GPDO (introduced August 2020). Here is what it allows and the conditions that apply:

What is allowed

Detached houses: You can add up to two additional storeys (if the existing house is two storeys) or one additional storey (if the existing house is one storey), provided the total height after the addition does not exceed 7 metres or the height of the existing roof, whichever is greater, up to an absolute maximum of 18 metres.

Semi-detached and terraced houses: You can add one additional storey to a two-storey house or one additional storey to a one-storey house, up to a maximum of 3.5 metres above the existing roof height and the total height must not exceed the highest building that shares a party wall or is adjacent.

Eligibility

The house must:

• Have been built between 1 July 1948 and 28 October 2018
• Be at least 3 storeys below (after the addition) for two-storey additions to detached houses
• Not be on designated land (conservation areas, AONBs, National Parks, the Broads, World Heritage Sites, SSSIs)
• Not be a listed building
• Not have been created by a previous change of use under permitted development (e.g., office-to-residential conversions)

What is not allowed

• Adding storeys to houses built before 1 July 1948 (most Victorian and Edwardian houses are excluded)
• Adding storeys to houses built after 28 October 2018
• Adding storeys to flats, maisonettes, or HMOs
• Adding storeys on designated land
• Engineering works to strengthen the existing structure are allowed, but the original external walls must be retained

Unlike most PD rights, adding an extra storey requires prior approval from your local council. This is a more detailed process than a standard PD check.

The fee is £249 per dwelling.

The council must assess:

• Impact on the amenity of neighbours (light, privacy, outlook)
• The external appearance of the building, including the materials
• Air traffic and defence considerations (relevant near airports)
• Whether the building is safe from fire (the fire service is consulted)
• The impact on protected views (if applicable)

Timeline: The council has 8 weeks to decide (or longer if both parties agree). If they do not decide within 8 weeks, approval is deemed granted.

Neighbour consultation: The council must notify all adjoining properties and consider any representations received.

Any upward extension needs building regulations approval. The requirements are substantial because you are adding significant structural load to the existing building:

Structure (Part A): The existing foundations and walls must be able to support the additional storey. A structural engineer will assess the load-bearing capacity and design any strengthening works needed. This is often the most expensive element.

Fire safety (Part B): A three-storey house has significantly stricter fire regulations than a two-storey house. You will need a protected escape route, FD30 fire doors throughout, interlinked smoke alarms, and potentially a sprinkler system. See our loft conversion building regulations guide — the fire requirements are similar.

Sound insulation (Part E): The new floor between the existing top storey and the new storey must meet sound insulation standards.

Thermal performance (Part L): Walls, roof, and windows must meet current insulation standards.

Staircase (Part K): A new staircase to the additional storey must comply with the same requirements as a loft conversion staircase.

Adding an extra storey is a major project. Indicative costs:

For a typical 50m² additional storey on a semi-detached house, expect a total project cost of £249,000–£200,000 including all professional fees.

Adding an extra storey is one of the most expensive home improvement projects. It makes financial sense when:

• The additional space would be extremely valuable (high-value areas where the cost per m² of additional space is less than buying a larger house)
• The existing house has strong foundations and structure that can support an additional storey without major strengthening
• The alternative (moving house) is more expensive when stamp duty, agent fees, legal costs, and the price differential are factored in

It is less likely to be worth it when:

• The existing structure needs significant strengthening
• The house was built before 1948 (not eligible under PD — would need planning permission)
• The cost of construction approaches or exceeds the value added

• Assuming your house is eligible. The date restrictions (1948–2018), designated land exclusions, and building type requirements rule out many properties. Check eligibility before investing in design work.
• Underestimating structural costs. Many existing houses need significant structural strengthening to support an additional storey. Get a structural engineer's assessment early — this is the biggest cost variable.
• Forgetting fire safety. Creating a three-storey house triggers much stricter fire regulations. Budget for FD30 doors on every floor, a protected escape route, interlinked alarms, and potentially a sprinkler system.
• Not considering neighbours. Prior approval includes neighbour consultation. An additional storey that overshadows or overlooks neighbours may be refused.