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Home & garden.

Updated: 30 Jun 2021

Loft conversions jargon buster

A guide to the key loft conversion technical terms. We'll help you understand the world of loft extensions and get to grips with common jargon.
Which?Editorial team
Velux window 436698

From the different types of loft extension to planning permission, our jargon buster will help to demystify the technical terms you're likely to encounter when planning a loft conversion. 

Whether you've decided to take the plunge and have a loft conversion, or you're still at the planning stages, you'll quickly find yourself in a world of confusing terms and industry jargon. 

We've rounded up some of the most common terms to arm you with everything you need to know.

Get help planning your loft conversion with our expert step-by-step guide and avoid annoying or costly mistakes by visiting our page on insider tips from experts and homeowners.

Builders finish

Typically, a builders finish involves fitting plug sockets, light fittings, skirting boards, architraves and plastering the walls. It's unlikely to include decorating, tiling, carpeting etc.

If your building firm says that it will complete your conversion to a builders finish, it's worth double checking exactly what's included.


A dormer is a type of loft conversion in which an extension protrudes from your existing sloping roof, usually at the rear of the property.

Dormers are the most popular type of conversion because they are relatively cheap to undertake compared with other types of conversion and are suitable for most properties.


Hip-to-gable conversions involve extending the peak of the sloping 'hip' roof at the side of the property outwards to create a vertical 'gable' wall and create more internal space.

To have a hip-to-gable conversion, you must live in a semi-detached or detached house, as the extension is made to the side of the property.

If you have a detached house with sloping roofs on either side, you can extend both to create a double hip-to-gable.


Mansard loft conversions run along the length of a property's roof and involve changing the angle of the roof slope to make it almost vertical.

They are the most expensive type of conversion, but are suitable for most properties and create a good deal of extra space. You can see examples of the different types of loft conversions on our loft conversion step-by-step page.

Party Wall Agreement

If your loft conversion requires you to carry out any work on the wall that joins your house to your neighbour's, you'll need to have a Party Wall Agreement.

This is a formal agreement between you and your neighbour that aims to ensure the work carried out won't endanger your neighbour's property.

You can find out more about Party Wall Agreements in our guide to building regulations and planning permission.

Permitted development?

Many houses have a built-in allowance for developments and extensions that can be completed without needing planning permission - known as permitted development.

Most loft conversions are considered permitted development and can be carried out without planning permission.


The pitch of a roof simply means the steepness of it.

Some types of loft extensions, such as mansard conversions, work by altering the pitch of the roof to create more space internally.


Rafters are the sloping beams inside your loft that support the roof.

If you have rafters, as opposed to roof trusses (see below), your loft should be easier to convert.

Roof trusses

Roof trusses are timber frameworks that span the cross-section of your loft space and support your roof.

Typically, roof trusses will make a loft trickier to convert than old-style rafters (see above) and could double the cost of your extension.

Shell conversion

Shell conversions are part conversions, where the structural work is carried out by a building firm, but the rest is left to you.

Usually, a shell conversion involves completing all structural timber work, creating a watertight internal loft space, fitting windows and doors, chipboard flooring, a staircase and any internal stud walls.


Velux is a popular brand of skylight that has become synonymous with rooflight windows.

A Velux conversion simply involves fitting Velux roof lights, and is suitable only if your loft is already the right size and meets building regulations.