Can I claim the use of home as office allowance if the work that I perform there is (currently) unpaid? I am currently working on a web project that I'm developing in my office at home. I'm spending a significant portion of my week working on the project, but it's out of hours (usually 4-6 hours per evening and some time on the weekend). So it's a high sum total of weekly hours, but it isn't my day job (I'm working in a contract for a client during the day) so I'm wondering whether I would be able to claim the allowance or whether it's simply a red flag that it isn't worth waving?
You don't have to receive wages to claim home office as a business expense, however, you do need to use the office exclusively for that business and not for personal purposes.
For the US, see this summary from TurboTax for more detailed information.
For the UK, as long as the office space has no other use for the time you are working you can deduct the expenses for that specific time frame. See this explanation from HMRC.