Dormer extension over low sloping roof

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Tamlyn

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I would like to know if it is possible to do an L-shaped dormer extension on a certain Victorian terrace in London. The issue is that the rear of the house, which contains the bathroom on the first floor, has a low sloping roof that cuts off the corner of the room (see photos).

mh-bathroom.jpg


mh-rear.jpg


My questions:
  • Would building an L-shaped dormer on this house be prohibitively expensive?
  • Would it be Permitted Development?
The way I imagine it could be done is to build up the brickwork to the level of the existing ceiling then build the dormer over that. However that would mean moving or removing the eaves on the side wall. I've read the PD technical guidance and it looks like that would not be allowed (section B.2 (b)(aa)).

Several houses on the street have done dormers but only over the main part of the house.

I'd appreciate any thoughts or experiences on this.
 
I suspect there would be problems with the window overlooking neighboring properties.
Also there would be problems with roof details, e.g. gutter between side of window and existing main roof, or access between side of window and existing main roof for maintenance, etc.
 
Assuming you are not listed etc, an L-shaped dormer on the rear is normally permitted development, subject to certain conditions, the main one being increase in volume, which should be no more than 40m³.

You also have to set the dormer front back slightly from the eaves, for both the rear-facing and side-facing dormers. This does increase structural complexity, but there are standard ways of dealing with this. It is important to leave some of the slating in at the eaves, and also to leave a little of the original roof in at the end of the outrigger. If you don't, it may not be regarded as a 'roof enlargement' (the rules don't refer to dormers!), but it could be regarded as an additional storey, coming under Class A, and therefore not permitted development.

Extending a dormer onto an outrigger does cause issues with some LPAs, and there are a number of appeal cases where decisions have gone either way. But if you are sensible with regard to overall appearance/size, you should have few problems.
 
Yes to both of the above I am doing exactly that on my property at the moment, under PD.
 
Don't do what your neighbour's neighbour has done at the party wall. Instead, build the party wall up properly in brick. This makes it much easier for your neighbour to build theirs when the time comes, rather than hacking away at yours.
 
Thanks for all the responses.

I'm struggling to visualise the profile of the dormer over the outrigger. Assuming the roof line is not altered, that means the floor of the dormer would be ~50cm above the eaves and the highest part of the brick wall on that side. How would the floor joists be supported?

@Himaginn happy not to have any side windows in the dormer. I didn't understand your comment about roof detail.

@Nakajo thanks for the tip re party wall and good sleuthing re location. Those (fugly) dormers on Forster road do look like they have the floor level above the top of the brick wall. Interesting.
 
You can have side windows. They just cannot be openable, and must be obscured below 1700mm above FFL.
 
Tamlyn said:
@Himaginn happy not to have any side windows in the dormer. I didn't understand your comment about roof detail.
Click to expand...
It's difficult to imagine how your main roof will meet a side dormer without a dormer on the main roof. Unless there is sufficient space left for access/maintenance.
 
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