Help how to claim proportion of utilities, council tax & rent as an expense on tax return

DrPepper1234

Hi all, I am a little confused on rules on this. We have one room in our home that is used predominately as an office (this is not used as a bedroom), my other half is a sole trader and his work is conducted away from the house but obviously has a certain amount of admin, phone calls etc that is conducted each evening from home. 

We realise we could use flat rate simplified expenses but feel working out costs of utilities, council tax, rent etc would work more in our favour. What we can't fathom is how to work that out, I am searching online but getting conflicting information.

For instance, the number of rooms...I know kitchens and bathrooms are not included, is it bedrooms plus living areas such as sitting rooms? We have a conservatory, is that to be included?

So once you have worked out the number of rooms I assume this number is divided by your total expense, leaving the cost for that one room, you then charge a certain percentage? I think its 20%, but as I have said, I have seen conflicting information? 

I hope above makes sense and someone can enlighten me. I have read that HMRC do not have clear rules on this, possibly because they prefer the simplified expenses? 

Thanks in advance 

Jeremy535897

HMRC view here:
https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim47800 et seq

The examples at https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim47825 are useful.

DrPepper1234

@ Jeremy535897 Thanks, not sure I can make head nor tail of that, but Ill have a better look later :-)