Loft conversion with existing extension

10 Mar 2022
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United Kingdom
Morning all,

We're intending to do a hip to gable loft conversion with dormer across the rear of a semi detached house, typical 1930's semi. We already have a 3m pitched roof single story extension at the rear of the property, the extension and the main roof are not linked, the extension roof touches the house below the upstairs windows. The LPA has said we need full planning as we exceed the 50m3 limit due to the entire volume of the extension being included in the volume calculation (so we're 22m3 over). Is this the correct interpretation of the regulations?
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I would qualify this by saying permitted development is not my strong point but the guidance for Class B which covers enlargement of the house by alterations or additions to the roof seems to state that the volume calculation only applies to the original roof and they should not include any extensions. Extract below:

(c) 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

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.

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 and 50 cubic metres for semi-detached and detached houses. Any previous enlargement to the original roof space in any part of the house must be included in this volume allowance. ‘Terrace house’ is defined in the legislation under the section ‘Interpretation of Part 1’ - set out at the end of this guidance. ‘Original roof space’ will be that roof space in the “original building” (see ‘General Issues’ section of this document above for the definition of this).

Good Luck
I think different LPA's apply different policies here, but why not just go in for full planning (depending on if your local planners hate dormers or not) mine do not though and you might be able to get a bigger dormer by going the full planning route depending how steep/large your roof is

I've experienced planning including roof volume from a side and back extension into the PD volumes.
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The 50 cubic metres is the additional volume over and above the existing volume of your roof.
If your LPA is including the new habitable space within the existing roof void, as well as the enlargement itself, they are mis-interpreting the rules.
If you have a single-storey extension, the roof of that should not be taken into consideration.
Hi All,

Thanks for the advice. I've gone back to the LPA with the information you've provided but they will not budge unfortunately. I found an appeal decision online that specifically states that roofs over single storey extensions should not be included (search for APP/W5780/X/15/3121479) but they still won't have it.

They still claim that "Any previous enlargement to the original roof space in any part of the house must be included in this volume allowance" covers any extension.

We are going to have to go to full planning which adds cost, hassle and may push the build back. I didn't want to go this way as PD should have given us all we wanted space wise.

Planning isn't my area of expertise, but I have observed that my clients generally think planning is a bigger deal than it really is, so I would just go for it.
One option is to just crack on and not worry about your LPA's misunderstanding of the rules. Another option is an LDC which will, presumably, be refused, and then will need to be appealed to the Planning Inspectorate.
Go for full planning and the loft conversion you want rather than one limited to PD specs. If it's granted-great. If it's knocked back, appeal (if there are any good grounds). If appeal fails then go back to your PD scheme and let them chase you, with that wording and the precedent they aren't going to win :)

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