Whats this trailer thing and how does it move?

[Captain Answer]

Noticed these a few times, usually on building sites but never had chance to get a picture of one before

What is it and how does it work? I wondered if the outside covers a trailer chassis or does it load onto a frame somehow like a container would?

[Rowe]

It's a welfare unit.

It'll either have wheels, or gets dragged or lifted onto the back of a lorry

[the tribester]

Chippindale Hire

[Captain Answer]

Thanks both, I did try googling various terms but couldn't find it

[fridgedoctor]

Yep, welfare unit.
Little kitchen, drying cupboard (useless, stinks of fumes) toilet, all powered by an inboard genny. The wheels and tow hitch bit are all on it, it does some hydraulic or wind up transformation once sited, but it gets towed in by a van or pick up.
Seems to have been designed to be not very comfortable. Crap really, and dear to rent too.