Hi bod

This is far from "hypothetical" - something that has happened 1000s of times since project notifications were required before anyone had even thought of CDM.

Somewhat to my surprise is is actually covered in the guidance, L153 that supports CDM 2015

48 If a construction project is not notifiable at first, but there are subsequent changes to its scope so that it fits the criteria for notification, the client must notify the work to the relevant enforcing authority as soon as possible.

On the assumption that the enforcing authority is HSE then I would avoid using the online notification system and instead send the required information (as specified in Schedule 1 to the Regulations) to HSE Bootle with an explanation that the project was not initially notifiable but now is. Reduces the chance of an HSE person asking difficult QQ!

In ye olden days pre CDM some contractor would get a job to build a house and would work out that they could do it in less than 30 days but then the rain came or e.g. the excavator broke down and the ridiculously overoptimistic programme got extended. As an HSE Inspector I was never impressed by the idea that the project had not been notifiable in the first place.