Volume calculation roof - advice required

[Frank2010]

Hi, new to this forum, not really by choice

I'm currently fighting a very lengthy battle with my Borough. It seems the conversion from hip to gable under PD has woken the mighty wrath of the local Planning Officer.
To make matters worse there is an argument whether the build has exceeded permitted development of 40 cu m (terraced property). To cut a very long story short my builder went ahead without getting a certificate of lawfulness (we were new to this all, he managed the application and build). We have now completed without the necessary paper work in hand and have been told that the whole structure may have to be removed, another conversation altogether.

There is some argument how the roof volume is being calculated and I was hoping whether anyone could clarify this for me.

The calculation for the box dormer to the back of the roof is:-
Width x Depth x Height divided by 2. This calculation seems fine and I'm in agreement with the officer to the approach.

The 2nd formula is for calculation of the hip to gable conversion:-
Width x Depth x Height divided by 6

Does anyone know whether this calculation should include the eaves of the house or not? The premise being that the eaves have not been altered in any way and are therefore part of the original structure and volume.

Can someone qualified clarify this for me please?

 

[DevilDamo]

If you apply for a Certificate of Lawfulness for a proposed development, then the planning fee is 50% of a normal householder fee, which is normally around £75.

If you apply for a Certificate of Lawfulness for an existing development, then the planning fee is 100% of a normal householder fee, which is normally around £150.

If for whatever reason your certificate is refused (i.e. on the basis the proposed works are/were not lawful, as in not falling within the PD guidelines), then you'd have to look at submitting a full planning application, but with the possibility of the application being refused. The planning department could then take enforcement action OR you go to appeal. Appeals can be a lengthy process and they do have their pro's and con's. A pro being it'll be a while before anybody from the LA asks for the unauthorised works to be removed. A con being it could cost you a lot of money and time to go through the process without a guaranteed successful outcome.

As for the roof calculation. The 40cu.m is for all "additional" volume that has been created so if eaves storage existed, which would now be within the new room(s), then that wouldn't be taken into consideration as it already existed. I have done the same for a huge loft conversion and I have recently been successful with the Certificate and proved the additional volume created was below 50cu.m (detached house).

I could have a quick look and carry out a quick calculation to see what figures I come out with if you want?

 

[Frank2010]

Thanks DevilDamo, that background info is very useful. I don't think an additional £75 will break the bank at this stage Considering what's at stake and fees have been paid and no one has complained so far.

We have already been told that a retrospective application would most likely be refused on grounds that in our area hip to gable is not encouraged and the roof/ sides not set in/down enough. Under PD, the guidelines have been followed (as loosely defined as they are) hence I think the planning officer is steering this in another direction. I've already made loads of counter arguments on all of this and really what it boils down to at the moment is the volume and a non obscured window, which we have immediately agreed to obscure/change etc. So the volume does seem the main issue the officer has got us on.

I should be more precise about the eaves. The officer is taking into account for her calculations the box running outside the house, holding the last set of tiles and where the gutter is fixed to. Effectively the area that sits outside the structure away from the walls and is original. Hence our argument that it was already existing and shouldn't be taken into account for calculations.
It might sound trivial but omitting this area or adding it makes a huge difference to the calculations.
The officer has calculated an area of 46.44 cu m, taking the box into account. My calculations without that area bring the build down to 40.32 cu m.
This is still outside permitted I know but we also had the problem that the builder made the structure slightly larger to original plans submitted. We are now on revision 3 so far.
However if we can prove that my figures are correct, we could argue that the original plans were lawfully submitted and that perhaps the breach is minor and not worth pursuing.
To further complicate the matter we have a sloppy engineer who did the drawings. On request of the planning officer he had to alter the plans as the structure had deviated from pictures taken. He altered the plans, put on physical dimensions but it seems left some of the drawings untouched.
The planning officer is ignoring the physical dimensions on plan and is scaling off plan with ruler, which also adds to the problem making the volume larger. Our engineer is willing to revise the plans yet again if need be but this won't make any difference to the volume anymore as all dimensions are on plan.

Bottom line. I need confirmation whether her approach is correct, taking the whole roof structure into account for hip to gable conversion. Or mine, just using dimensions for the area that was changed.

DevilDamo. I would take you up on your kind offer but not sure what different calculations you would come up with. The formulas used were supplied by the planning officer and that's the baseline she will be working to. It's now really a simple case of punching in numbers. It's deciding which numbers to punch in that's causing the issue

 

[simonjay]

This seems a little confused / confusing. There should be seperate before and after drawings produced and the volume taken. The volume includes the eaves and all parts of the roof. The volumes should be subtracted to work out additional volume.
Also its not clear which eaves is being discussed.
Presumably where the hip is replaced with a gable, the eaves will have been removed along that area.
If there is an eaves in front of the new dormer, presumably this has not changed (or has it been enlarged ?)

However, the best way to prove volume changes without doubt is before and after drawings with calcs. Then tell the planners to prove the drawings wrong. You don't fuss about this bit added this and that bit removed that !!

Unfortunately using the scale ruler things is how they work, and the plan drawer should have known this. Revised drawings may be required.

Hope that helps,
Simon.

 

[DevilDamo]

I don't think an additional £75 will break the bank at this stage

I thought you said the works have already been carried out, which in that case, a full planning fee (i.e. £150) would be required, not £75. The £75 fee is for "proposed" works, as in... works that are yet to be carried out.

Effectively the area that sits outside the structure away from the walls and is original. Hence our argument that it was already existing and shouldn't be taken into account for calculations.

Well if it already existed, the volume should be used in the existing AND proposed calculations, which would provide an increased volume of 0cu.m as it already existed.

The formulas used were supplied by the planning officer and that's the baseline she will be working to. It's now really a simple case of punching in numbers. It's deciding which numbers to punch in that's causing the issue

I would have just put together some sections and calculated from there, which is what I did with the job I'm involved on. You do know that the Planning Portal has a tool in working out roof volumes?

 

[Frank2010]

Thanks for all your help guys and I agree with everything you say/ suggest.

This is really difficult to explain as it hinges on a very fine detail and interpretation.

I need to find answers as to what figures to use for the planning officers hip to gable formula. My surveyor is also not sure why the planning officer is making the calculations as she does, so it's our word against hers. I'd like confirmation whether our thinking is right or wrong before I push this down a lengthy argument, potentially not knowing what I'm talking about.

I've attached a quick illustration, perhaps you guys could be so kind and provide views on this, as this forms the basis of our defence, that her calculations are wrong.

The planning officer is calculating the area including the eaves, the box outside the structure under last set of tiles where the gutter is attached to. In my illustration this is the whole area in large triangle marked A and from the outside does indeed look like the complete roof area.

The rooms downstairs go into that triangle, i.e. we have curves at the end of the ceilings on the first floor and the rooms push into the roof area if this makes sense. It is almost a square as illustrated in B.
So the disputed area is effectively the area marked in red. Our argument is that this area is already existing, wasn't developed and forms part of downstairs. As such she shouldn't include it in her calculations and should only calculate the area which makes up the smaller triangle in A.
As her calculations take the whole area into account this pushes up the volume by the height of the eaves x area of the whole house.
She will argue that this area does form part of the development.

Is my argument flawed and the disputed area should be taken into account or do we have a solid defence to take this further? Or put it simple when does a roof area become part of the downstairs?

I appreciate this sounds perhaps a tad picky but this level of detail may count at the moment. And without an independent view I don't feel confident enough to push on this. My surveyor is obviously backing this argument for personal reasons to ensure the blame doesn't end with him or he may just be right, I can't say.

So any views appreciated, fire away .....

Untitled

• Frank2010
• 19 May 2010
• 1

 

[simonjay]

Presumably you have the gable in line with the wall of the house, so along that side you have effectively removed the eaves box volume. So you cannot just use an "increase in volume" formula - there is added volume in one area and removal from from another.
It is not relevent how the internals are divided up. You have to take into account the whole house.

It would seem that her method is flawed. She can use the formula from the bottom of the eaves, but must then remove the assumed vertical slice of gable that is not there since the overhang has been removed.

Or, she can use for formula from the bottom of the triangle formed between pitch and gable, but must subtract the eaves box removed from the bottom of the gable.

The increase in volume of the remaining side eaves due to the hip-to-gable obviously will be included as extra volume.

If this is what you are saying, I agree with you !

As I said, you need to do before and after calculations, and then a subtraction. Then challenge them to show the error in your calculations. If need-be get the calculations signed off by a mathematican !

Simon.

 

[jeds]

What Simon says here is correct. Measure the roof as original and then as enlarged. It is the difference that counts.

It would help if you could post a photo of the extensions.

 

[Frank2010]

Presumably you have the gable in line with the wall of the house, so along that side you have effectively removed the eaves box volume.

Correct

She can use the formula from the bottom of the eaves, but must then remove the assumed vertical slice of gable that is not there since the overhang has been removed.

I believe she may have done this. The calculation/ length used for the width from hip to gable stops at the flank wall, so does not include a non existent box.

Or, she can use for formula from the bottom of the triangle formed between pitch and gable, but must subtract the eaves box removed from the bottom of the gable.

I'm struggling to get my head round this one.

The increase in volume of the remaining side eaves due to the hip-to-gable obviously will be included as extra volume.

Just about able to make sense of that.

As I said, you need to do before and after calculations, and then a subtraction. Then challenge them to show the error in your calculations. If need-be get the calculations signed off by a mathematican !

Thanks guys, makes complete sense. I'll try this approach tonight instead of trying to work out what has been added. It may be easier to calculate the volume as it stands minus the build as it was. The shapes are easier and I can probably make use of readily available volume calculators.

Keep you posted what figures come out at. If there are anything close to what I had calculated originally I may go the route of having it signed off by someone more qualified. I'll also see whether I can take some pictures in between as it's easier to explain that way.

 

[pradeep39]

hi, i am very new to this but i would like to do a hip to gable loft conversion with a rear dormer. will this be possible due to every other house in the near vacinity having a hipped roof? will i need to get planning permission?

 

[DevilDamo]

Start your own thread and members can advise accordingly, instead of using somebody elses