A house or an outside enclosure maybe with a house in it?
For just a house, many of us repurpose an old dog house and place a rubber flap on the door to keep out drafts and rain.
Go as big as you can is the general thought.
You can't recreated a natural tortoises range.
So start at say 3 meters square. 8 to 10 feet. Four times that size...Even better!
What is your location?
Do you have a secured yard or garden? Dogs? Neighborhood kids?
Predators?
You may need something with a wire top.
Keep in mind that tortoises can climb up such a fence and injure themselves. Either by falling down are getting their leg stuck. A 13 year old tortoise is unlikely to have very many enemies in the UK (beware of dogs), so an open top enclosure might be option as well. In any case do make a smooth wall so the tortoise can't climb out or reach the fence.
What do you mean with put in it? Are you referring to plant species?
check out www.thetortoisetable.org.uk there are a list of plants you can use to put in the outdoor enclosure. Also, plants that are sprayed with pesticides will also need a quarantine period. They should be washed off and planted in new soil. This allows the plant to wear off and get rid of the pesticides.
All this is done so your tortoise wont be consuming plants coated in harmful pesticides.
BUT if you are able to find a nursery that does not treat their plants with pesticides, then that would make things easier