
I would get a local and reputable builder, one preferably that comes by recommendation, to come and look first to see what they think and talk over ideas. It will give you an idea of a ball park figure you might be looking at.
Then you will need to get plans drawn up if the planning office say you need planning permission.
You will need a good architect to do this and also structual plans maybe required which have to be done by a structual engineer.
You will then apply for planning permission. Builders will not really be able to give a quote until the plans have been approved as there maybe changes involved which would mean revising the quote.
The easiest way to get funding is get your parents to get a top up on their mortgage as I doubt anybody will fund the extension for you as the property isn't in your name.
You will then have to add at least another 20 % to the quote for all the added extras so as not to have any surprises.
It is a big thing to do and a big commitment as the property isn't in your name.
Is renting not an option as this will be a better bet for the first year as it will give you time to get all your finances stable and it means you haven't got to commit to anything rather than a six month tenancy agreement which is better than a 25 yr loan which I would think you would have to get to fund a project like that :)
HTH
Lucy
xx