Permitted develop Dormer loft, then 2storey side and rear extension rules

3 Oct 2012
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United Kingdom
Been using the site for several years and pickup up good advice.

I am planning to put a hip to gable box dormer under PD (semi=50m3). Also I intend to put a part 2 storey and part single at rear first and at a later stage a 2storey side (maybe).

1. "Development is permitted by Class B subject to the following conditions:

(ii)other than in the case of an enlargement which joins the original roof to the roof of a rear or side extension, no part of the enlargement extends beyond the outside face of any external wall of the original dwellinghouse; and

Interpretation of Class B

B.3 For the purposes of Class B, “resulting roof space” means the roof space as enlarged, taking into account any enlargement to the original roof space, whether permitted by this Class or not."

My understanding is that the 2 storey side and 2 storey rear roof constructions would not count towards the 50m3, as the the dormer and the reconstructed roof would only be on the original house, or is am I getting it wrong way round?

2. The rear part 2 storey part single to the original house would have a flat roof? as it would not be possible to construct a 'v' shape sloping roof originating from the original roof since the box dormer is in the way?

I have been through the site and other sites, and I getting conflicting messages as some say, do the loft dormer first (PD), other places, I read, do the extensions first, then do the loft under PD. If I did the rear extensions first, I would have to redo the roof for 2 storey to a flat roof.

Your ideas highly welcome, Many thanks
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I do not know the planning rules in your area but most do not allow flat roofs on two storey extensions so this is the first point to establish. See if your council has planning guidelines on their website.

I would say that you need to build the rear extension and dormer separately. If you build your two storey rear extension first, when you build your PD dormer you will need to deduct the volume of the rear extension roof from the 50m3 allowance. If the two storey extension has a flat roof then it will not have much, if any roof volume. A pitched roof might only have a volume of 10-15m3 but will need to be calculated.

If you build your dormer first under PD and then submit a planning application for the rear extension there is a risk that they could refuse it as they might not like the way that the rear extension roof joins the dormer. In my area the planners require a 1m gap between a dormer and a two storey roof extension so if you build a full width dormer and then try to add a rear extension this could be a problem.
If any part of the dormer touched any part of your side or rear extension (which it almost certainly would) the whole lot would need planning permission.
I’d just get planning permission for the lot and do it all together.

We bought a semi detached house with a flat roofed double extension on the side and then converted the loft, put a dormer on and extended the hip over the flat roof.
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Why not build the 2 storey rear extension first, with sloping roof, flat on top. This will create a much bigger loft space than your dormer. Then either add a few velux, or apply for a couple of mini dormers.

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