Your tortoise should be able to get from indoor to outdoor area without human assistance to enable them to have an independent life. They can be taught to use a cat flap quite easily which will help to ensure the heat is kept in the shed.
The area should be as large as possible, preferably the whole garden, and should be escape proof. Tortoises (particularly Hermanni) love to climb, so the perimeter should be of walling/fencing with a minimum vertical height of 18" or 12" with an overhang. Most tortoises also like to dig so you need to ensure that there is a barrier beneath the fencing in case they dig deep. You can also use bricks, concrete or slabs to try and prevent them digging under the fence.
Avoid growing any climbing plant on an outside wall that might give the tortoise a foothold. It is best if the soil is light, well-drained and infertile (ideally chalk).
An area of grass should be allowed to grow until the weeds have flowered and seeded, then the grass should be cut and the hay removed. This encourages the growth of weeds which the tortoise will eat.
There should be some hard surfaces like walking stones or small patio area. If there is not enough hard surface, the claws of the tortoise will grow too long. If there is too much the claws will wear away, so ensure that you have a variety of substrates.
The whole area should be as sunny as possible but sheltered from the wind. A south facing garden is perfect as it gets the sun all day, but then ensure that you also have some shrubs so that they can shelter or aestivate if they wish.
There should also be rocks and changes of levels to provide interest and exercise, but these should be arranged so that the tortoise cannot fall onto a hard surface. A PEN on the lawn eg. rabbit run, is NOT acceptable.
You need to have an Indoor Area leading onto an Exercise Area and a Planted Area.
If you have a pond then you must ensure that it is securely covered or a barrier put up like log rolls or bricks, but you must ensure that they cannot get into it as tortoises WILL drown if they get into a pond.
If you have a steep area like a rockery then you either need to ensure that they cannot get onto it at all, or change the layout so that they can climb over it or up and down it safely.
You must ensure that all areas are secure by using proper fencing. A wire fence will stop your tortoise but will also cause damage to your tortoise as they will keep trying to get through it. They can rip scales or toes off which will end up with you having to spend money on vet's bills. You can use proper made fencing bought from a garden centre, have it made especially for you or you can get your own posts and planks and make your own.
The main things that you have to remember is that tortoises are a bit like sheep, if they can see through it they will go through it.
Be aware that tortoises can also smell rotting wood so make sure that you regularly walk around the fencing and check it for any rotting parts. If they can smell it they will pick away at it until they get through.
To preserve the wood you can use wood preserver and also if you put the bottom of the wood onto gravel this also helps to prevent the wood from rotting as quickly.
You also need to ensure that your tortoise is safe from extreme weather, such as flooding. So if you live close to a source of water then please keep an eye on the weather otherwise you may have a shock. The photo below is not where the tortoises are kept but this needs to be considered when making a tortoise area.
photos by kind permission of L Jones.