What I am typing is exact, its not opinion.
A mobile home cannot be built on site, it has to be built by a British Standard registered manufacturer and has to be built "inside" there are 2 BS that you are allowed to build too one is holiday standard the other being residential standard.
A mobile home must be on a towable chassis whilst a temporary building does not, they are very different items.
You cannot extend a caravan yourself but you can refit it both internally and externally onsite (but not all in one go)
Personally I would buy 2 cheap caravans 10' wide join them by way of a central walkway and cover it, this will give you a great boot room, drying area, utility room. Then externally clad the vans with Timber, replace windows for double glazed, buy a log burner with back boiler then refit them out completely how you want them.
Having a wood burner with rads is brilliant and it doesn't matter that we lose some heat from the roof because the wood is free. Insulate underneath, then try and make airtight skirting.
Our 2 vans cost 1500 log burner 250 rads 160 walkway I made from pallets roofing 200, timber cladding 670, insulation 200ish we have now been in ours 4 years, my son wants a bigger room so we are going to do a small reconfiguration this autumn.
Hope this helps
If you have any further questions please do not hesitate to ask.