The two notification thresholds
Under CDM 2015 Regulation 6, a construction project is notifiable if it meets either of these conditions:
The construction work will last longer than 30 working days and have more than 20 workers on site at any one time.
The construction work will exceed 500 person-days in total.
If either threshold is met, the project is notifiable. You do not need to exceed both. A small project with three workers running for 170 working days hits 510 person-days and is therefore notifiable under Threshold B, even though it never has more than 20 workers on site.
What counts as construction work?
CDM 2015 defines construction work broadly. It includes building, alteration, fitting out, commissioning, renovation, repair, upkeep, redecoration, external cleaning with water or abrasive, demolition, dismantling, and the installation, maintenance, or removal of mechanical, electrical, gas, compressed air, hydraulic, telecommunications, and computer services that are normally fixed within or to a structure.
Site preparation, exploration, investigation (but not mineral extraction), and the clearance or preparation of a site for use or for building work also count. If in doubt, assume it counts.
Who is responsible for notifying?
The duty to notify sits with the client. Under CDM 2015, the client is the person or organisation for whom the construction work is carried out. The client may delegate the practical task of completing and submitting the F10 form to the principal designer, a project manager, or another competent person, but the legal duty remains with the client.
For domestic projects where the client does not appoint a principal designer, the duty to notify transfers to the contractor (or principal contractor, if more than one contractor is involved).
When to submit the F10
The notification must be submitted as soon as practicable before the construction phase begins. There is no fixed deadline (such as 14 days before), but in practice the earlier you notify, the less likely you are to receive an enforcement notice for late notification.
The F10 should be updated if the particulars change materially during the project, for example if the principal contractor changes or the expected duration increases significantly.
Consequences of not notifying
Failure to notify a notifiable project is a criminal offence under CDM 2015. The HSE can issue improvement notices or prosecute. In practice, the HSE inspector will usually ask for the F10 during a site visit. If the project is notifiable and no F10 has been submitted, the inspector will issue an enforcement notice and may consider further action depending on the circumstances. The fine for non-notification is unlimited in the magistrates' court.
Common edge cases
Phased projects cause the most confusion. If a developer splits a large scheme into phases with different contractors, the entire scheme counts as one project for notification purposes if it falls under a single client. Each phase does not reset the person-day count.
Maintenance contracts with rolling call-offs can be notifiable if the total person-days across the contract period exceed 500. A facilities management company running a team of four for a year-long maintenance programme will exceed 500 person-days and should notify.
Temporary structures, scaffolding, and site welfare facilities all count towards the workforce and duration totals.