How is a UK UBO calculated through layered company structures?

Walk the company's PSC tree and multiply the stakes along each chain. The test in the Money Laundering Regulations 2017, regulation 5(1)(b), is more than 25% of the shares or voting rights, held directly or indirectly. Companies House publishes stakes in bands ("more than 25% but not more than 50%", "more than 50% but less than 75%", "75% or more"), so the careful approach is to use the lower end of each band.

Worked example: ParentCo holds 75% or more of MidCo, MidCo holds 75% or more of TargetCo, and a person holds 75% or more of ParentCo. At the lower ends: 75% × 75% × 75% = 42.2%, more than 25%, so the person is a beneficial owner of TargetCo.

The other way: ParentCo holds just over 25% of TargetCo and a person holds just over 25% of ParentCo: about 6.25% indirectly, so not a beneficial owner through that chain. They may still be one through another chain, so every path has to be walked and added up.

Control counts on its own: regulation 5(1)(a) and (c) cover ultimate control over management and other control, such as the right to appoint or remove most of the board. Companies House records these as "natures of control" on each PSC.

In practice: walk to five levels, watch for loops (company A owning company B owning company A), mark owners registered outside the UK as needing their own country's records, and screen every beneficial owner against the sanctions lists, and for PEP. Stratum's company and owners check does the walk and the screening together; the Companies House Watch API's ownership tree (/v1/ch/companies/{number}/psc-tree) gives you the walk alone.