Self employed working from home using office plus another room constantly for storage

The information on how to either claim simplified or calculate oneself for an office at home for a sole trader is easy to find. However there is no information about how to claim for in addition to this using a significant space constantly in a garage or room for storage, The question is how should one properly calculate and claim for this - additional - constant storage use of part of the home?

Hi,
You would need to find a way to apportion costs that your business incurs in that room.  
If you cannot do this, you cannot make a claim.  
Please have a look at https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim47815.
Thank you.