What home office expenses can I actually claim as a sole trader?

You've got two options and you pick whichever gives the better result. HMRC's simplified expenses let you claim a flat monthly rate based on your hours: £10 a month for 25-50 hours, £18 for 51-100 hours, or £26 for 101+ hours — no need to measure rooms or work out bill proportions. The catch is that the flat rate doesn't cover broadband or phone costs, which you'd claim separately based on business use. If your actual costs are high (say you heat a dedicated office all day), working out the real business proportion of electricity, heating and so on might beat the flat rate. What to avoid: don't claim for the whole of a bill when there's personal use too, and don't claim both the flat rate and actual costs for the same thing — it's one or the other. GOV.UK has a simplified expenses checker that compares the two methods side by side.