Converting Your Loft – Key Structural Elements

This 3rd loft blog looks at the key structural elements that will form your new loft. It is these elements and their interaction which will create the space in your loft.

The key elements that create the structure are;

New Loft Stairs

New stairs will be required for your loft conversion. Under building regulations the maximum pitch for new stairs is 42 degrees, all the steps have to be the same height and you need a minimum of 2m head height over the stairs. (There are some minor relaxations where the head height for new stairs to a loft conversion is limited.) These regulations will determine where the new stairs can go.
The location of the new stairs is critical, where the stairs finish in the loft will determine how well the new loft space works. In locating the new stairs the aims should be to; prevent the loss of any existing living or bedroom space in the floor below, to maximise the space created in the loft and to make the rooms in the loft work as efficiently as possible. To achieve this the stairs should ideally start on a landing or hallway and finish where you can create a landing with direct access to the rooms in the loft.
If the new stairs follow the line of the existing stairs this will be the best use of space. If you copy the details of the banisters and balustrades it will look as though the stairs have always been there. If your property has Permitted Development (PD) rights and you build a dormer, mansard and/or a hip-to-gable you should be able to achieve this.
If your loft conversion is subject to planning approval you may have to locate the new stairs under the ridge to satisfy building regulations. You might be able to get planning approval for a small dormer into which you can fit the new stairs. You will need a dormer with an external width of about 1.8m to achieve this.

New Floor Structure

The floors of loft spaces are only designed as ceilings. Ceilings are normally constructed from 50 by 100 joists (4” by 2”) and are not strong enough for new habitable rooms. You will need new deeper floor joists, which means the finished floor level will be higher than the existing.

New Roof Structure

Unless you are building a dormer, mansard or a hip-to-gable the existing rafters will remain in place. To maximise the internal space any purlins or bracing will be removed and replaced with a timber dwarf wall (about 800mm to 1m high). To ensure structural stability the remaining rafters are likely to need beefing up. Adding thicker rafters and the required thickness of insulation (100mm or so from the face of the existing rafters) will mean the existing roof is reduced in height.


A dormer will determine where the new stairs go. It can significantly improve the space and layout in your new loft, making the difference between a cramped space and one that feels like the rest of your home. Dormers usually have a timber structure and a flat roof but you can also have a factory produced modular steel framed dormer.


Mansard loft conversions are commonly seen in London where mansard roofs have been a feature since the Georgian times. A mansard roof has two sloping elements each side, the lower roof is at a steeper pitch than the upper. A mansard conversion involves raising the party walls and building a structure in between the walls. Mansard roofs do not give quite as much space as a dormer but are considered to be more attractive on period buildings.


Hip-to-gable conversions work well on properties where the existing stairs run along-side the gable end wall with a hipped roof above the gable (1930s semi-detached properties often have a hipped-end). A hip-to-gable would allow the new stairs to follow the line of the existing, it would give the required head height for the new stairs and allow space for a landing at the top of the stairs. A hip-to-gable combined with a dormer can create a really big loft conversion.

Converting the loft of the Rear Outrigger

A rear outrigger is an original extension at the back of your home at right angles to the main part of the property. If you have a loft over your rear outrigger and your property has PD rights converting it is a smart way to significantly increase the floor area you can create in your loft conversion.

External Materials

The finish and materials for a loft extension (a dormer, a hip-to-gable and/or conversion over the rear outrigger) will be determined either by the planning approval or by the PD rules for your property.
If planning approval is required your choice of materials will be limited by Council policy and by the type and status of your property. You may be required to match the existing materials or your Council may be more flexible.
If your property has PD rights the PD rules state that “the materials used in any exterior work shall be of a similar appearance to those used in the construction of the existing dwelling house.” Council’s may interpret this differently so check with your Local Authority.

The structures you can add to your existing loft will create the new loft space. What you can build will depend upon either planning approval or the PD rights for your property. When proposing big roof extensions under PD the PD rights can be quite complicated. If in double, seek professional advice and apply for a Certificate of Lawfulness for your proposals to ensure you do not breach planning regulations.

For details of my other loft blogs please check out my website:

Please note this is a guide and is not a definitive source of legal information.

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