The various requirements which need to be met are set out below. I also set out my particular techniques for meeting those requirements but while these are effective they are not the only way. You will find other resources on the internet that offer other, equally good, ideas. Please, however, ensure that you accept what are only proven ideas such as might be evidenced by consistent success in breeding. Also please remember that even within as small a country as the UK a suitable outdoor vivaria in one part might not work as well - or at least with the same species, in another.
Orientation. In this part of the world almost all herps need a site which includes a sheltered but sunny southerly exposure in order to maintain body temperature. During the summer, they will also use east-facing slopes in the morning and west facing slopes in the afternoon. As a natural consequence an East-West orientation is preferred, ideally with a slope facing towards the south. While our garden actually has an east facing slope the vivaria are terraced up the garden and each has then been the subject of the necessary earthworks to create sunny and sheltered exposures. Note in the simple diagram aside, the vivarium itself faces south and there are slopes (in brown) facing in all directions except north. The pink area is a hollow wall and the two yellow areas open sand.
Drainage. While many amphibians are unconcerned about drainage, the vast majority of reptiles prefer a well drained habitat where they can be sure the soil is not too damp. Even amphibians will thrive in the same conditions so long as they have access to water - especially for breeding. People who live on sandy soil will not need to worry further about this. In our case, however, we live on fairly heavy soil which can be very moisture retentive over a layer of chalk. As a result, every single outdoor vivarium starts as a hole at least 45 cms deep with about 15 cms of packed hardcore at the bottom of it. Although some of our local soil is replaced over this a good proportion is replaced with a mixture of peat and sand. This ensures that the habitat does not become waterlogged.
Escape proofing. This can be achieved in many ways. Generally, however, the vivarium is bounded by some sort of wall, constructed of a not too easily climbable material with a substantial overlap at the top. It is also essential that the wall itself extend deep enough below ground to prevent the animals from digging out. Many lizards and some amphibians are extremely accomplished burrowers and, especially if you have a light soil the depth of the wall would need to be significant. In our case, the retaining wall is actually built on a concrete base using the perimeter of the drainage hardcore as a foundation. Since the wall extends down to the hardcore the risk of digging out is negligible. The wall itself is built of lightweight concrete blocks laid flat and raising to a height of between 20 cms and 60 cms (reflecting the slope of the garden) above ground. Above this there is a glass wall in aluminium framing between 30 and 60 cms high which is topped off by inverted plastic guttering. Last, but not least, when furnishing the vivarium one must ensure that neither plants nor structures are near enough or tall enough to allow animals to jump from them to the top of the wall.
Predator proofing. Most of us live in close proximity to other people which means that domestic cats are a major potential predator. In our case we have two cats of our own. One of them would happily sit in the vivarium watching the lizards while the other would try to catch and kill every one of them! Some birds can also be potential predators. How this is addressed is very much related to the design of the vivarium. A small garden vivarium could simply have a lightweight timber frame covered in netting over the top. Obviously if you use this approach then you must ensure it is easily removable for access and yet is also fairly firmly secured the rest of the time Also, of course, you have to ensure that the interior decor is not high enough to allow animals to reach the netting and use this as an escape route. Where, like us, the vivaria are large the whole assembly can be enclosed in what might be described as a large fruit cage, keeping predators out of the entire area while allowing access via a gate. In our case this is built of reasonably substantial timber covered in wire netting which withstands cat's claws! There is also danger from burrowing predators such as moles which will kill and eat lizards in hibernation. Fortunately our design prevents this due to the packed layer of hardcore for drainage.
Decor and landscaping. This is dependent on the species one wishes to keep. For amphibians, for example, the vivarium would need to have a good sized pond. Even for lizards a pond is beneficial as a water source in extremely dry weather. The interior could, as with our largest vivarium, be sculpted to give east, south and west slopes surrounding a pond. We have also added a wall with a hollow interior which provides both a popular basking place and additional hidey holes within it. Vegetation should so far as practical, reflect natural habitat. In the UK, and much of Europe, prime habitat is heathland and as a result the plants that predominate in our vivaria are Erica and Calluna the two major heather families. A combination such as this will actually suit the majority of European herps.
Hibernaculum. A hibernaculum is a place where animals can hibernate. While many of the animals will be only to happy to dig their own burrows some prefer them to be ready made - in the wild they might use rodent burrows for example. During the construction phase it can be a good idea to construct a simple shelter of lightweight blocks lined internally on the sides and top with expanded polystyrene. A drainage pipe can be laid to provide access and the whole thing covered in the landscaping stage with just the end of the drainage pipe emerging for access. All of our vivaria have something like this within them. It must be remembered that the animals must be able to climb in the access pipe. In the case of smooth pipes such as plastic drain pipe, coating the inside with diluted exterior PVA adhesive, sprinkling sand on it and allowing it to dry will achieve this.
Appearance. Lastly, while not necessary to the animal's well being it might be necessary to bear in mind that other people might be able to see the vivarium and, quite simply, don't want to see something ugly in the garden. Obviously it might be wise to consider this in the design stage. We have made very effort to make our vivaria, if not things of beauty, at least not eyesores! You should find some pictures in the gallery.
Summary. An outdoor vivarium must cover all but the last of the above factors. If the last is given proper consideration as well, it might be the final factor that swings things in favour with your partner of you being able to build an outdoor vivaria in your garden. Hopefully, the above notes will give you some idea of the factors to be taken into account, the problems that might be considered and ways in which you might address them. So ........ start digging!