If it's essential that you convert the outbuilding you should check out the chances of getting planning permission before completion.

Are the outbuildings close to the house (eg within the "garden") or further away (eg clearly in the agricultural part). That might affect your chances. In the long term it would be financially best to try and get PP to convert them as "seperately occupied dwellings" rather than as "Granny annex" or some other use "incidental to the main house". This is because at some point in the future you might wish to sell or give the outbuildings to the daughter/grand daughter, or perhaps even someone else. It may seem unlikely now but worth considering.

It's likely you would also need PP for "change of use" for the workshop unless you can show it's been used for something similar for at least 10 years (I forget the exact period but it's longer than the normal 4 years).

You should also check the deeds for covenants that might prevent you doing all this or make it too expensive.

You should also get all the buildings surveyed and tell the surveyor. If any of the buildings are in poor condition you need to know if they will have to be demolished and rebuilt rather than converted. This is important because getting planning permission for demo and rebuild it much harder. As soon as a building is knocked down the planners treat the replacement as a "new building in the countryside" which is against their religion.

Check out the availability of services. For example just because the house has electricity doesn't mean you can automatically build and connect two more houses and an office. Might need a new transformer or something. Ditto the telephone and drainage if applicable.

Good forum for everything selfbuild here..

http://www.ebuild.co.uk/cgi-bin/forums/discus.pl

Essential reading:

"How to get planning permission" and "How to find and buy a building plot" both by Roy Speer available from Amazon.

You may have to finesse you planning application for residential conversion, given the absurd ruling by the Welsh Assembly Government that agricultural buildings may only be converted to 'economic units' - it. workshops, holiday lets and the like. You may have to hint to the planners that you're developing a holiday let rather than come clean and say it's for residential purposes. There is an exemption for 'affordably homes' and for special cases, but I suggest you have a quiet word with a local architect before committing yourself to the planning process.
It's a ridiculous ruling, and one which many of us in Wales are lobbying to overturn.

My strategy is to give no hint of intentions, make no prelim enquiries to Planners. In secret, work up a superb scheme and exceptional presentation, leave no stone unturned to mark yourself out as very different from the average dross the planners are faced with. Fit in with the rules to the extent of finding out what all the boxes are and ticking them very clearly, or if not, justifying clearly why not, hopefully in the wider public interest. But don't expect anybody incl the Planning Officers (with honorable exceptions) to read more than a dozen words of anything before making up their mind! Make the application and simultaneously have a campaign to personally present the plan to everyone - neighbours, local amenity group, local green/energy/Transition Town groups, Chairman of Parish Council, District Ward Councillors including Planning Committee Members, Council sustainability liaison Member etc. Then even if they oppose it at least they'll be opposing the real thing, not some version they've made up. You'll soon find out everyone's bizarre prejudices! Councillors are often worse than Planning Officers, in wanting to rigidly enforce 'guidelines' as gospel. Just make yourself look as different as possible, determined, justified and unstoppable. Meet the Planning Officer on site, just keep insisting on the common-sense rightness of your plan, wait, and maybe he'll just give an inch and then you'll know which way to make that a mile, compromise and at least get a foot-in-door Consent, then come back for more. If it's not working, try to get a Councillor to get it 'called in' to be considered by full Committee, not decided by Delegated Officer power. Have someone honest, competent and likeable address the Committee for the allotted 3mins - the Committee can vote to over-rule the Officer's recommendation. But be prepared to go to Appeal, and for that hire the most hard-boiled elderly gamekeeper-turned-poacher Planning Consultant, who will charge you a lot for one sheet of killer A4 and laconic presence at the Hearing (you can tell I have someone in particular in mind!). The consultant won't want to get involved until you've achieved a Refusal, but he may just give you a valuable hint of the key principle to plug, in your Application. At all times be clear that your scheme is a no-brainer for the public good - any scheme should in fact be that. Good luck!