You may find it useful to get some measured plans of the existing building and site and any proposed new layout. That will make it easier to specify what you want.
Do you want to improve insulation, ventilation, heating, hot water, solar, cooling etc? These are worth considering early on. £250k sounds a healthy budget but windows and external doors, kitchens, landscaping, automatic gates, wrap around balcony and glass balustrading can eat through a lot of cash.