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.

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This is No5 in a series of 7 videos

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.

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Addition or alteration to roof of house

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.

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Skylights are 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.

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Roofspace for 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.

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Original roof space

Resulting roof space means the roof space as enlarged, taking into account any existing or proposed extensions, whether permitted by this Class or not.

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Resulting roof space

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

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Max increase for terraced

or 50 cubic metres for other house types

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Max increase for other types

External Limits

The roof enlargement cannot overhang the outer face of any external wall of the original house.

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Overhanging not allowed

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.

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Minimum setback

The eaves of the original roof are to be maintained or reinstated.

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Eaves of original roof

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.

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Restrictions do not apply here

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.

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Facing materials

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.

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Principal elevation indicated by front door position

This is the existing roof slope of the principal elevation and it fronts the highway. No extensions are allowed beyond this plane.

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Existing roof slope

A dormer window can be added to the rear elevation under PD rules.

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Dormer window allowed under PD

The roof hips can be extended to gable-ends to allow a larger roof extension.

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Larger dormer on gable-end roof

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.

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Ridgeline perpendicular to highway

Dormer windows can be built on the other three roof slopes.

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Dormer windows allowed on 3 slopes

In fact, a more practical solution might be to create one large dormer roof, ensuring it is lower than the original roof height. 

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Large dormer is more efficient

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 ….

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No roof slope on the principal elevation

… or the entire pitched roof can be replaced by a Mansard-type roof.

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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.

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Obscure glazing for windows on side elevations

Openable windows need to be at least 1.7m above floor level.

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Openable windows

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.

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Rooflights covered by Class C

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 …

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Principal elevation fronting the highway

… architectural features on a non-fronting elevation may dictate that it should be the Principal Elevation

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Architectural features

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.

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Not fronting the highway

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.

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Dormers require 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.

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Full-length dormer windows

If the hipped roof has a ridgeline perpendicular to the highway, dormers can be built on all 4 roof slopes.

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Ridgeline perpendicular to the highway

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.

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Full-length dormers

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.

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The Outbuildings video is now ready for you!