Garden room lantern roof - apex/dual pitch for PD?

[PhilboSE]

Currently planning a garden room and having experienced some standard <2.5m height rooms I am keen to get more internal room height whilst being compliant with PD. Due to fortuitous ground levels and/or the ability to locate the room more than 2m from a boundary I can take advantage of higher roof limits.

Question is: does the inclusion of a roof lantern transform the roof from a single pitch to a dual pitch and thereby lift the max height from 3m to 4m? I can imagine that a tiny lantern in a large flat roof is incidental, but a very large lantern with just a small flat area around it might be considered the roof. I have done lots of reading of the regs and searching but can't find anything covering this somewhat specific question.

Is there a point at which a roof lantern becomes a lantern roof?

[Equus]

PhilboSE said:

Question is: does the inclusion of a roof lantern transform the roof from a single pitch to a dual pitch and thereby lift the max height from 3m to 4m?

No.

The actual wording of the GPDO specifies 4 metres for a dual pitched roof and 3 metres (depending on distance to boundary) for any other case.

Unless the entire roof surface is dual pitched it's an 'any other case'.

At risk of becoming boring by repetition: design what you want, then decide whether it needs Planning Permission. It's crazy to compromise a design for the sake of a £231 planning application fee.

The amount of time and ingenuity people waste trying to dodge through loopholes beggars belief.

[PhilboSE]

Equus said:

PhilboSE said:

Question is: does the inclusion of a roof lantern transform the roof from a single pitch to a dual pitch and thereby lift the max height from 3m to 4m?

No.

The actual wording of the GPDO specifies 4 metres for a dual pitched roof and 3 metres (depending on distance to boundary) for any other case.

Unless the entire roof surface is dual pitched it's an 'any other case'.

OK, thanks for the clarification. It was the element of what constitutes a dual pitched roof that eluded me.

Equus said:

At risk of becoming boring by repetition: design what you want, then decide whether it needs Planning Permission. It's crazy to compromise a design for the sake of a £231 planning application fee.

The amount of time and ingenuity people waste trying to dodge through loopholes beggars belief.

Not quite the situation that you assumed. Project started off as a bog standard garden room well within PD, but now that I'm actually doing the "design what you want" bit, we are considering something a bit grander but should still remain well within PD. The main reason not to go to planning is actually to save the time, as I am doing this as a self build and want to get started next month rather than in November.

[Equus]

PhilboSE said:

Not quite the situation that you assumed. Project started off as a bog standard garden room well within PD, but now that I'm actually doing the "design what you want" bit, we are considering something a bit grander but should still remain well within PD. The main reason not to go to planning is actually to save the time, as I am doing this as a self build and want to get started next month rather than in November.

You can submit a partially restrospective application and carry on regardless, at your own risk.

There's a box on the application form that asks whether any part of the works have already commenced: simply tick that.

[PhilboSE]

Many thanks for the information, Equus. Will continue with the design work and will then see if PP is required.

[TA14]

I think that it's a neat idea and you could detail so that the lantern is the whole roof. However, whilst it may give a commercial edge to a manufacturer in part of the market, the time to design the structure and the manufacture of specific components would exceed the £231 many times over. On the time basis, are you going to build the base and the walls in the hope that PP comes through before you put the roof on? Or build a temporary flat roof and replace that with a permanent roof designed to comply with PP or permitted development depending on the result of the PP application?

[PhilboSE]

I think I can keep under the 3m height limit without affecting the design and therefore not need PP. A sketch of the current design is this:



I'll be building it in traditional timber frame style with a breakfront over the central French doors then dressing the front with some timber lintel and pilaster details to pick up on a few elements from an orangery on the main house. The main house is a chunky Georgian thing so this design will sit better than a timber clad traditional garden room. My plans for this one is to put aquapanel cladding on the exterior and go with a lightweight render which will make the room look like it's built more substantially and also be zero maintenance.

My wife was pushing for a bigger lantern roof which would then be marginal on staying within the 3m limit but I think that it's overkill. The actual flat roof element will be canted back towards the rear so that the eaves height meets the PD issue. I guess this last issue is one that might push me into going for PP as it would be slightly nicer (and easier build) to have a flat ceiling inside without worrying about the 2.5m eaves height.

[Pheo]

Presumably I’m right in thinking that if building a flat roofed Garden Room within 2m of the boundary, I can’t stick a normal roof light on it without increasing the roof height for planning?

Not sure if it would lift the consideration of eaves?

[PhilboSE]

It wouldn't change the consideration from where the eaves are calculated but it would change the highest part of the building, which if >2.5m would not be PD.

(Eaves are calculated from lowest part of the topside of the roof directly above where it meets the outer wall).

[Pheo]

Ah, I thought that was eaves height. Gotcha.

[Equus]

PhilboSE said:

(Eaves are calculated from lowest part of the topside of the roof directly above where it meets the outer wall).

And interestingly the rules exclude parapet walls, so for the design shown above, the 'eaves' height would potentially be calculated to the invert level of the concealed perimeter gutter.

You could, therefore, have a perimeter gutter with an invert level 2.5m. above ground, then step the rest of the flat roof height up to the 3m. limit.

Here's one I did earlier - for an extension, not an outbuilding, so the eaves was limited to 3m. (within 2m. of a boundary) and the lantern was allowed to be 4m. high. The 2966mm. measurement is the 'eaves' height, according to the technical guide:

[Blib]

Apologies for butting in, OP.

We are contemplating something similar in style, but smaller (3.5m x 3m) to square off the back of our cottage

Is there a percentage limit in the building regs for how much roof area can be taken up by the lantern? Or, can you make it any size that you like? Possibly taking up most of the roof area?

Thanks.

ETA: I've just seen TA14's reply

Edited by Blib on Saturday 15th August 07:40

[Equus]

Blib said:

We are contemplating something similar in style, but smaller (3.5m x 3m) to square off the back of our cottage

Is there a percentage limit in the building regs for how much roof area can be taken up by the lantern?

Yes...sort of. In your case you're talking about an extension, not a free-standing garden room?

If you are subject to Building Regulations, and if you have more than 25% of the floor area as glazed area, then you're going to need calculations to prove the thermal efficiency requirements are met (and will need to compensate for the additional heat loss through the excess glazing by improving the specification elsewhere).

Conversely, if your roof lantern is over 75% of the roof area, and you meet the other requirements (at least 50% of external walls formed of transluscent material, not including walls within 1 metre of boundary, single storey at ground level and thermally separated from the main dwelling), then it's classed as a conservatory and is exempt B.Regs.

Edited by Equus on Saturday 15th August 07:58

[Blib]

Ah!!! Thank you, Equus. You're so generous with your time and expertise on this forum.

The two external walls will be of timber framed glazing and the french doors to the living room will be the thermal separation.

Go big it is!!!

[PhilboSE]

Equus said:

And interestingly the rules exclude parapet walls, so for the design shown above, the 'eaves' height would potentially be calculated to the invert level of the concealed perimeter gutter.

Indeed, this is a sketch of the design that I am working to:



The existing ground level is in green and is higher at the rear of the site than the front, so by digging down and building a retaining wall and then building against that, I can get an extra 300mm height internally, and I can go up to 500mm higher than the eaves for the top of the lantern.

Building off an insulated slab and using 220mm roof joists the head height internally should be about 2500mm, which is plenty, and much much more than what the turnkey garden room builders are offering using ground screws or piles: