Notification to HSE :-

You should complete an F10 form when you think you are undertaking a project that will last more than 30 days or involve more than 500 person days.

http://www.hse.gov.uk/co...cdm/faq/notification.htm

Almost ALL construction work will come under CDM --as

Part 2 covers general duties that apply to all construction projects
Part 3 contains additional duties that only apply to notifiable construction projects, ie those lasting more that 30 days or involving more than 500 person days of construction work
Part 4 contains practical requirements that apply to all construction sites

CDM applies, but would normally be under threshold for notification. Biggest element of the job will be putting in the pads (foundation). Running cable shouldn't need to be a huge job. May not need water and drainage depends where it is in relation to main office or whatever. Once the thing has been offloaded and linked up get someone else to check the electrics before you occupy it. I've come across three cabins with no earth connection and one with a live earth... I've worled with guys who've worked three or more years in removable structures, they can be quite comfortable when properly set up. Plan it properly, furnish it and move in.

I am taking this that as you work in Food and Drink, that this is an office/warehouse environment, and you just want another office. I dont personally THINK (didnt check), that this needs to be notified, and unless there is a toilet its doesnt need water, a simple water dispencer until can be suitable. (Again all I think's!)