volume confusion regarding hip to gable loft conversion

[jim88]

Hello all

ive been in with my architect today regarding a hip to gable loft conversion and they've come back and said its exceeded the 40m3 allowance (victorian end terrace)

From my understanding I can't increase the space of the roof by 40m3. Just for context the property is 8m wide and its 4.8m ridge to eaves. My plan was to just do a hip to gable conversion and keep the dual pitch of the roof with Velux either side but because my 1st floor ceilings are 3.1m heigh I plan to reduce them by 600mm to accommodate much more room in the loft however the architect is saying the area gained by dropping the floor joists will contribute to the 40m3 allowance. I had no idea this was true, is it? I would have thought it would have been calculated from the new space gained outside the roof line etc. What if someone did a hip to gable conversion under 40m3 and then a few years later decided to drop the 1st floor ceilings, would they fall foul of permitted development?
thank you in advance to anyone who takes the time to shed some light on this

 

[freddiemercurystwin]

You're correct it doesn't include the internal space only the increased external space.

 

[^woody^]

IIRC, the "old" guidance mentioned something like "additional volume of the additional roof", implying it was just the additional extended roof volume, but a quick scan of the current guidance says

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 (which could imply the enlargement created by the floor being dropped, which could be done without planning permission)

To be permitted development any additional roof space created must not increase the volume of the original roof space of the house by more than 40 cubic metres for terraced houses ...... Any previous enlargement to the original roof space in any part of the house must be included in this volume allowance. (which could imply the enlargement created by the floor being dropped)

‘Original roof space’ will be that roof space in the ‘original building’ (see General issues for the definition of this). (the original roof space before the floor was dropped)

 

[wessex101]

I agree with the OP and Freddie.
I would apply for a Lawful Development Certificate and show the volume calculation for the existing roof and additional space created by the hip to gable conversion. I wouldn't even mention lowering the existing ceiling as that has nothing to do with Planning.

 

[Jamess19888]

Would I/architect use external or internal measurements for the calculations.

Thanks for everyone's input so far

 

[Jamess19888]

What measurements do P and Q relate to

 

[freddiemercurystwin]

Are you the same person as the OP?

 

[jim88]

yes logged in from an old account on my Mac, my phone was a different account

 

[wessex101]

What measurements do P and Q relate to

P is the length of the ridge but you shouldn't need that figure for a hip to gable.
Q is the length of the hipped section of roof.

There are 2 calculations for the hip to gable volume, using the strange annotations on that drawing -
(O x Q x W)/6 or 1/3 ( Base x Height) Where Base = ½ (O x W) and Height = Q Both using the external dimensions.

EDIT: Sorry I made a mistake, the letters on that drawing are really confusing, now corrected.
EDIT 2 in red

 

[Jamess19888]

If I get all the measurements, is some one able to do me a drawing demonstrating the volume increase, something that the council will accept when I apply for the ldc. Obviously I will pay a fee. Please message me if this is something you can do

 

[freddiemercurystwin]

Surely your architect should be doing that as part of the LDC?

 

[jim88]

well my architect is saying they are well over 40m3 allowance without even accounting for the dropped ceiling which doesn't seem right. I had a dormer and hip to gable put on my old house and it didn't exceed the allowance. On this house im only looking for a hip/gable conversion, not even a flat roof dormer, hence why Im questioning my architect

my measurements are

house width 8300mm
house depth 9600mm

house ridge length 5000mm
house hip to (outside of proposed gable wall) 3.3m

internally in the loft the height from top of 1st floor ceiling joists to ridge is 2.5m

im looking for an online calculator to try work this out, would I need any other measurements

 

[freddiemercurystwin]

Well over 40m3? Give me a minute .....

 

[freddiemercurystwin]

Check the dimensions but for the increased external volume it comes to 13.2m3.

 

[wessex101]

I make it 13.2m³
I spotted another mistake on my previous formula, it should be 1/3 (Base x Height) not ½
EDIT: Great minds think alike!
EDIT 2: It makes you wonder what the hell this so called "architect" is doing?