Measuring Loft Allowance Cubic Volume Increase

ChloeRadshaw1 · 25 Sep 2017

I would like to do a hip to gable on both sides of a detached house as well as a rear full width dormer. The neighbour has done it

I have used the calculator here https://www.planningportal.co.uk/info/200216/volume_calculator_pitched_roof_two_end_gables

But this only allows the hip to gable to be calculated and not the rear dormer. Additionally I cannot even use this calculator

The house is square with depth of 10M and width of 10 and the highest roof pitch is 2M.

How can I calc this easily>

ChloeRadshaw1 · 25 Sep 2017

From using the online calc it looks like I will be creating 450 cubic metres !!!

HeightMetres = 3.5
WidthMetres = 10
LengthMetres = 10
Roof HeightMetres = 2

Total Volume Cubic Metres == 450

tony1851 · 25 Sep 2017

It looks like you've worked out the total volume of the roof?

A rough sketch showing basic dimensions - including approx. length of the existing ridge - would help.

Nakajo · 25 Sep 2017

Provide a plan, section, and elevation (existing and proposed), and I suspect someone here will tell you the volume

chappers · 26 Sep 2017

can't be sure of you're current volume due to the hips but your new dormer once raised to gables is going to be 5mx10mx2m=100m3 so half of that alone is going to be your 50m3 increase and that is before taking into account the raising of the gables. You will almost certainly be into planning territory if that is what you're angling at.

ChloeRadshaw1 · 28 Sep 2017

chappers said:
can't be sure of you're current volume due to the hips but your new dormer once raised to gables is going to be 5mx10mx2m=100m3 so half of that alone is going to be your 50m3 increase and that is before taking into account the raising of the gables. You will almost certainly be into planning territory if that is what you're angling at.

How have you worked that out? On the basis that the rear dormer will take half the size of the house? That is just wrong maths because that disregards the distance from the eaves and the fact that some space is already there

Ian H · 28 Sep 2017

If your dormer went from ridge to eaves and was a cube measuring 10 x 5 x 2 that's 100cu/mtrs.

Assuming your roof slices that in half you can disregard half of it (which would be inside the existing roof space), that leaves you with a 50cu/mtr dormer.

If your roof is only 2.0mtrs high I don't think you could do it anyway.

tony1851 · 28 Sep 2017

ChloeRadshaw1 said:
How have you worked that out? On the basis that the rear dormer will take half the size of the house? That is just wrong maths because that disregards the distance from the eaves and the fact that some space is already there

He might not be far out - you said you were doing hip-to-gables, which will add considerably to the volume of the dormer itself.

chappers · 28 Sep 2017

ChloeRadshaw1 said:
How have you worked that out? On the basis that the rear dormer will take half the size of the house? That is just wrong maths because that disregards the distance from the eaves and the fact that some space is already there

Not sure what you mean by half the size of the house, it's about increase in volume of the roof space. then it's as Ian and Tony have said, as I explained originally.
Why are you wanting to calculate the volume of your whole house.
Your Dormer will when finished be 10 meters wide by 5 meters deep by 2meters high. you can disregard the front volume as that remains unchanged, you then have to subtract the volume of the back roof that is already there, so half that volume, less the volume you will be adding by turning your hips to gables. So you will definitely be over the 50m3 allowed under PD

Nakajo · 28 Sep 2017

I'd sure love to see a drawing; even one drawn by a non-professional. Also, I don't trust those calculators as they seem to refer to external volume, when PD expressly refers to 'additional roof space'.

Ian H · 28 Sep 2017

Nakajo said:
Also, I don't trust those calculators as they seem to refer to external volume, when PD expressly refers to 'additional roof space'.

Is that right? I paid someone to design mine at 50cu/mtrs and he ****ed it up and did it at 55 but that is probably external size.

Nakajo · 28 Sep 2017

Yep. Development is not permitted if... the cubic content of the resulting roof space would exceed the cubic content of the original roof space by more than (i) 40 cubic metres in the case of a terrace house, or (ii) 50 cubic metres in any other case;

garyo · 28 Sep 2017

Nakajo said:
Yep. Development is not permitted if... the cubic content of the resulting roof space would exceed the cubic content of the original roof space by more than (i) 40 cubic metres in the case of a terrace house, or (ii) 50 cubic metres in any other case;

And by 'roof space' would that exclude any inaccessible voids created by carcassing out walls in front of eaves etc?

Ian H · 28 Sep 2017

garyo said:
And by 'roof space' would that exclude any inaccessible voids created by carcassing out walls in front of eaves etc?

I just thought that myself but it says original so probably not.

But if a dormer roof is 6x3mtrs and 250mm thick then that's 4.5cu/mtrs off my dormer, that's near enough for me!!!

Nakajo · 28 Sep 2017

If you present the numbers that way, I very much doubt that an LPA would take issue with it.

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Measuring Loft Allowance Cubic Volume Increase

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