Dormer loft conversion cost: the box, its width, and what each line of the quote buys

How dormer loft conversion cost works

1. Settle whether it is permitted development. A great many dormer conversion are permitted development, which means no planning application, but the limits on depth, height, materials and how much of the plot is already built on are specific and a designer should read them against your own house before anybody prices anything.
2. Get drawings and a structural design. Existing and proposed drawings, then a structural engineer's calculations for whatever is being opened up or carried. On a dormer conversion this is a small share of the total and it is what turns an idea into something three builders can price identically.
3. Serve the party wall notices in time. Work on or near a shared wall, or excavation within three or six metres of a neighbour's foundations, engages the Party Wall etc. Act and the notice periods are set in months rather than weeks. Serving late is the commonest reason a start date slips.
4. Price it against a specification, not a sketch. Ask each builder to price the same written specification, with provisional sums shown as provisional sums. Two quotes that differ by a third almost always differ in what is included: the kitchen, the flooring, the decoration and the drainage diversion are the usual absentees.
5. Agree stages, retention and building control. A staged payment schedule against completed work, a small retention until sign-off, and a clear statement of who is notifying building control. The completion certificate is what a future buyer's solicitor will ask for.

Insurance and paperwork

Common questions

Is a dormer permitted development?

A rear dormer often is, within volume limits and set back from the eaves, while a dormer on the front roof slope normally is not. The limits also differ for a house and for a flat, and designated areas remove permitted development rights altogether.

Does the dormer roof matter?

It does. A flat roof on a dormer is a roof like any other and it should be specified properly, in a single ply, EPDM or GRP system with a stated fall and a proper upstand where it meets the tiles. That junction is where dormers leak.

Will a full width dormer look wrong?

It can, and some councils resist it for that reason even where it is technically permitted. A designer who has worked on your street will know what has been passed nearby, which is worth more than a general answer.

How much does a dormer loft conversion cost?

It depends most on the width of the dormer, which is why a full width rear dormer is a different job from a single box. The box is a small building with its own structure, flat roof, insulation, window and weather detail where it meets the tiles, and each of those is a line in the quote.

What should a quote for dormer extension cost include?

The same written specification priced by every builder, with provisional sums shown as provisional sums, and the dormer's own lines set out: its structure, a flat roof system with a stated fall and a proper upstand where it meets the tiles, the insulation and the window. Add a staged payment schedule and a small retention until building control has signed the work off.

Registered contractors by region

• Registered roofers in the East Midlands postcode area
• Registered roofers in the East of England postcode area
• Registered roofers in the London postcode area
• Registered roofers in the North East postcode area
• Registered roofers in the North West postcode area
• Registered roofers in the Scotland postcode area
• Registered roofers in the South East postcode area
• Registered roofers in the South West postcode area
• Registered roofers in the Wales postcode area
• Registered roofers in the West Midlands postcode area
• Registered roofers in the Yorkshire and The Humber postcode area