Marysunshine whatever planning regulations are in relation to doing up an existing building and whether your caravan is mobile or not, if the intention is for you to have the structure, mobile or not, as your permanent home that puts a very different slant on what you plan. You will need planning permission, you certainly will not be allowed a separate entrance (I assume you mean, car access or footpath). Your plan is fraught with problems.
I am definite on this because our next door neighbour is trying to do exactly the same thing - turn a run-down implement shed into a bungalow for, in his case, his daughter and family to use when visiting from the USA. The outside of his building will be unchanged, no extension or anything. He is in the fifth year of trying to find a solution that satisfies council and neighbours. He cannot have a separate foot or vehicle entrance for the property, he cannot have a chimney. The objections from neighbours and so on have been endless.
Then there are all the problems that would arise if you went into care and needed to release the funds you have invested in the property to pay for care.
To give a simple example. You sell your house for £200,000 and buy the property you both intend to live in and your son gives £100,000 towards your costs. If you go into care, the council will investigate your circumstances and will see that you spent £200,000 for the property and your son only paid you £100,000 and they will assess you as still having assets of £100,000 invested in the property. Either your son would have to sell the house when you went into care to pay for it, or they will put a charge on the property and he will have to stump up the £100,000 plus interest when you die.
I am afraid I wouldn't consider doing what you intend to do, in a hundred years.