Precedent for measuring loft conversion volume internally rather than externally?

lshapedloft · 17 Sep 2019

We are about to undertake a loft conversion on a terraced house which has the traditional Victorian layout with an outrigger. The outrigger is quite deep (7.5 m) which means that we are able to max out the 40 m³ allowance.

I’ve been reading all over the Internet about the way this is supposed to be calculated (internal versus external), and can’t seem to see a consensus.

In the external corner we have:

most permitted development plans submitted (when I looked them up on our local councils site for certificate of lawful development applications)
The planning portal calculator here says all measurements must be external: https://www.planningportal.co.uk/info/200212/volume_calculator_dormer_window_with_flat_roof

In the internal corner we have:

The legislation itself (http://www.legislation.gov.uk/uksi/2015/596/pdfs/uksi_20150596_en.pdf) speaks about “roof space”: B1(d) says “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”
“Roof space” would typically refer to the space inside the roof. It doesn’t say “roof volume”.
Threads like this: https://www.diynot.com/diy/threads/calculating-loft-volume-and-applying-for-ldc.508370/ (in particular Woody’s assertion)

I want to go with the internal calculation (obviously - we can build a bigger dormer) but as we are about to start the build (very soon) I am going to be doing this before getting a certificate of lawful development. So I’m bearing the risk that the council come round and tell me to shorten the outrigger dormer, at some great expense.

So - question: does anyone know of any precedent where this has been successfully defended as referring to the internal volume rather than external? Or any examples of certificates of lawful development granted where this has been the case?

tony1851 · 17 Sep 2019

The Planning Jungle website does not appear to have any appeal cases which support your assertion that it is the internal volume that is to be measured.
Granted the wording is not particularly concise, but I think you will find that the concensus among LPAs is that it is external volume of the enlargement which is taken into account.
I suggest this is also a logical approach because people could build dormers with double stud walls to keep the internal volume at 39.9m3, and on completion remove the internal 'layer' of wall to give themselves a much bigger dormer.
Before starting work, why not apply for an LDC based on internal volume and see how the council responds. If they refuse your application, take it to appeal and see what an inspector decides; this is probably the only way you will find out for yourself.

Leofric · 18 Sep 2019

tony1851 said:
If they refuse your application, take it to appeal and see what an inspector decides

But do they really want to get involved with an appeal for a loft conversion Tony

^woody^ · 18 Sep 2019

tony1851 said:
The Planning Jungle website does not appear to have any appeal cases which support your assertion that it is the internal volume that is to be measured

Do they have any that contradict it?

tony1851 · 18 Sep 2019

Leofric said:
But do they really want to get involved with an appeal for a loft conversion Tony

In practice, probably not.
But OP was concerned to squeeze the maximum from the space available and wanted precedent to see if his idea is correct, and personally I haven't come across any.

tony1851 · 18 Sep 2019

^woody^ said:
Do they have any that contradict it?

From a superficial reading of the relevant part, there doesn't seem to be any appeal case which decides one way or the other - though I might have missed it?

Leofric · 18 Sep 2019

lshapedloft said:
So I’m bearing the risk that the council come round and tell me to shorten the outrigger dormer, at some great expense.

Yes, you are.

lshapedloft said:
The planning portal calculator here says all measurements must be external:

Note on taking measurements

All dimensions must be based on external measurements of the building or structure involved and not internal measurements.
Do you really want to risk it ,

feeling lucky

lshapedloft · 22 Sep 2019

Thanks for all your input - we've decided to play it safe and use the external measurements as I could find no precedent of anyone using internal measurements (whether accepted or challenged) - so it feels like something somewhat left-field anyway. If we had more time I might like to do the lawful dev certificate first based on internal measurements and see if they challenge it, but with no time we're going for 39.99 m3 based on externals. It'll also mean we won't excite any jealousy (and associated hassle) on the street by having an outrigger dormer a (rather obvious) ~1m longer than anyone else's.

Nakajo · 23 Sep 2019

...other than those who applied for planning permission

lshapedloft · 23 Sep 2019

Our council isn't a fan of loft conversions in general - when we approached for pre-planning (we were doing planning permission for a ground-floor extension at the same time so threw in a >40m3 loft conversion) we were advised that planners won't go >40m3, and will often not even want to do that much, so that if we want to do 40 it would be best just to go with PD.

Leofric · 23 Sep 2019

lshapedloft said:
we were advised that planners won't go >40m3, and will often not even want to do that much,

Are you saying your local council has a figure limiting the maximum volume for a loft conversion even if you are applying for planning permission

lshapedloft · 24 Sep 2019

Not specifically. Just that when I went for pre-planning with a plan that included a >40m3 loft (alongside a GF extension) they said something to the effect of "there's no way you'll get that loft (unless it is *highly unusual* and architecturally significant, and then you may have a shot), and we don't really like the 40m3 ones anyway, so you'd be better to just do PD".

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Precedent for measuring loft conversion volume internally rather than externally?

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