Rules for building Loft Conversions using Permitted Development
Transcription of the video
In this video, I go through the rules for building loft conversions using Permitted Development.
This is video No 5 in a series of videos that started with Video No 1 considering eligibility for Permitted Development. Video No 2 listed the 10 conditions applicable to all extensions.
It is recommended that you watch videos 1 and 2 before you watch this one.
Class B provides PD rights for loft conversions, in other words, for the enlargement of a house consisting of an addition or alteration to its roof.
This includes the addition of one or more dormer windows and the transition from a hipped roof to a gable-end roof. Class B does not cover the installation of skylights as this is covered by Class C.
PD rights are only available to the original building or as it was on 1st July 1948. This means that PD rights are not available to raise the roofspace on an existing extension.
Maximum additional volume
There is a limit on how much you can enlarge the original roof space. Original roof space means the roof space in the original building.
Resulting roof space means the roof space as enlarged, taking into account any existing or proposed extensions, whether permitted by this Class or not.
The resulting roof space must not exceed the cubic content of the original roof space by more than 40 cubic metres for a terraced house
or 50 cubic metres for other house types
The roof enlargement cannot overhang the outer face of any external wall of the original house.
Roof extensions should be set back at least 20cm from the original eaves, measured along the roof slope from the outside edge of the eaves, guttering not included.
The eaves of the original roof are to be maintained or reinstated.
None of these 3 conditions, Overhang, Setback and Eaves, apply to a hip-to-gable enlargement or an enlargement which joins the original roof to the roof of a rear or side extension.
The materials used to construct the roof extension must be of a similar appearance to those used in the construction of the exterior of the existing house. Flat roofs of dormer windows will not normally have any visual impact and so dormer roofs can be covered with felt, rubber, zinc or any suitable compound. The materials used to face a dormer, including window frames, should appear to be of similar colour and design to the materials used in the main roof of the house when viewed from ground level.
Fronting the highway
No extension is allowed beyond the plane of the existing roof slope of the principal elevation that fronts the highway. This particular condition allows several interesting configurations for dormer windows. I’ll start with a conventional configuration for a detached house, with a front door indicating the principal elevation. The hipped roof has a ridgeline parallel to the highway.
This is the existing roof slope of the principal elevation and it fronts the highway. No extensions are allowed beyond this plane.
A dormer window can be added to the rear elevation under PD rules.
The roof hips can be extended to gable-ends to allow a larger roof extension.
Now lets consider a hipped roof whose ridgeline is perpendicular to the highway. There is an existing roof slope of the principal elevation which fronts the highway and so no extensions are allowed beyond the plane of this slope.
Dormer windows can be built on the other three roof slopes.
In fact, a more practical solution might be to create one large dormer roof, ensuring it is lower than the original roof height.
If the house was originally built with gable-ends, there is no roof slope of the principal elevation fronting the highway. This means that there is no condition excluding extensions beyond the plane of the roof slope. Two full-width dormers can be added ….
… or the entire pitched roof can be replaced by a Mansard-type roof.
However, for the dormer windows on the side elevations to be feasible under PD rules, they need to comply with the condition that requires obscure glazing in extensions that are on side elevations. Openable windows need to be at least 1.7m above floor level…. or the entire pitched roof can be replaced by a Mansard-type roof.
Openable windows need to be at least 1.7m above floor level.
If clear glazing is required, rooflights can be installed under Class C of the PD rules. Most loft conversions will need to be approved by Building Control. This will require the extension to conform with requirements for escape from fire, amongst other conditions.
Not Fronting the Highway
The phrases “Principal Elevation” and “Fronts the Highway” have particular meanings and they are discussed in video No 2: All Extensions. Two of the main points are … A house has only one Principal Elevation, usually fronting the highway, and …
… architectural features on a non-fronting elevation may dictate that it should be the Principal Elevation
With these points in mind, lets consider a detached house that does not have its principal elevation fronting the highway. In this scenario, the condition excluding an extension beyond the plane of the roof slope is no longer operative.
We’ll now consider 4 configurations. With a hipped roof and the ridgeline parallel with the highway, it is possible to add dormer windows on any or all of the roof slopes. If this is the principal elevation, these are the side elevations and these windows will have the obscure glazing.
If the hipped roof is converted to a gable-end roof, two full-length dormers can be added or an entire Mansard roof can be built.
If the hipped roof has a ridgeline perpendicular to the highway, dormers can be built on all 4 roof slopes.
If the hipped roof is converted to a gable-end roof, two full-length dormers can be added or an entire Mansard roof can be built. In this instance, the main glazing of the dormer windows does not require obscure glass, making it a far more attractive option.
Thats it for this video
In the next one, video No 6, I’ll go through the rules for adding outbuildings, rooflights. porches etc under Permitted Development.