Party Wall
Apr 30, 2024

How should I carry out a Loft Extension?

There are different types of loft extensions, their costs, and considerations as follows...

There are different types of loft extensions, their costs, and considerations as follows:

1. Simple Dormer Extension:

  • Most common and affordable.
  • Creates a box shape out of the rear slope of the loft.
  • Avoids planning permission with permitted development.
  • Cost: £30,000 – £40,000.
  • Additional bedroom and possibly an en-suite.
  • Duration: 3-4 months.

2. L-Shaped Dormer Extension:

  • Similar to a simple dormer but includes an extension over the rear outrigger.
  • Cost: £50,000 – £80,000.
  • Offers double the space for slightly more cost.
  • Duration: 3-4 months

3. Mansard Loft Extension:

  • Aesthetically pleasing, involves raising party parapet walls in brick so the neighbour can connect.
  • Planning permission likely necessary.
  • Cost: £35,000 – £75,000.
  • Duration: 4-5 months.

4. Hip to Gable Conversion:

  • Found in end of terraced, semi-detached, or detached properties.
  • Involves extending the inwardly sloping roof to create a vertical wall.
  • More expensive than mid-terraced dormer extensions.
  • Cost: £40,000 – £60,000.
  • Duration: 4-5 months.


  • consult with a good architect to maximize space within your budget.
  • determine which options require planning permission.
  • obtain planning/conceptual drawings and drawings suitable for Building Regulation approval.
  • involve an engineer for loading calculations, especially for steel beams.
  • consider the Party Wall Act if the property is not detached.
  • inform neighbours in advance for a favourable response.

For leaseholder, a loft space valuation is required and a premium must be paid to the freeholder. Typical premium for a London 2-bedroom flat worth £500,000 might be est. £40,000. Freeholders also often insist on a licence to alter to protect their interests and surrounding flats. Contact our team to organise advise by a party wall surveyor and/or a Registered Valuer.