Why there is no free HMRC route
The real choice is not accountant versus free, because the free route does not exist for non-residents. Almost every return filed from abroad needs the SA109, the residence supplement where non-resident status, split-year treatment, the personal allowance claim and treaty positions are all declared, and HMRC's free online service does not support it. GOV.UK directs non-residents to three routes instead: a paper return, commercial software that supports the SA109, or a tax professional. For the 2025/26 return, paper must reach HMRC by 31 October 2026, while a software or adviser filing has until 31 January 2027. See why the SA109 cannot be filed online for the background.
The trap: claims nobody makes for you
The expensive DIY errors are silent ones. Nothing on the SA109 applies automatically: split-year treatment must be claimed under the correct case with the right dates, the personal allowance (worth up to £12,570 of tax-free income in 2026/27) must be actively claimed by British citizens, EEA citizens and residents of many treaty countries, and treaty relief on pensions or other income must be invoked. A return that overpays sails straight through HMRC processing, and the lost relief stays lost until you notice and reclaim it yourself.
How to decide
DIY with SA109-capable software is genuinely fine when you were clearly non-resident for the whole year, have one straightforward UK income source and are repeating a year that was already filed correctly. Use an adviser in years of change: your first year abroad, a UK property sale, multiple income sources or a possible treaty or FIG regime claim, because a single missed relief can cost several times a typical fee. The full comparison, including what each route costs and a side-by-side table, is in DIY Self Assessment vs an accountant for non-residents.