The HSE has published a simple guide for small builders working in domestic construction projects.

It can be found on: http://www.hse.gov.uk/pubns/cis80.pdf

When working for a domestic client, the principal contractor will normally take on the client duties (to ensure the project has a plan for managing health and safety risks) as well as their own as principal contractor. If a domestic client does not appoint a principal contractor, the role of the principal contractor must be carried out by the contractor in control of the construction phase. Alternatively, the domestic client can ask the principal designer to take on the client duties (although this must be confirmed in a written agreement) and the principal contractor must work to them as ‘client’ under CDM 2015.

Source: http://www.hse.gov.uk/construction/cdm/2015/principal-contractors.htm