Shape: Many wildlife gardening texts recommend that a pond should have wavy margins, and from the aesthetic point of view, such ponds appear more "natural". Irregular outlines help increase the effective circumference of the pond, and hence the extent of shallow-water marginal habitats. It is likely that these will indeed help increase the number of micro-environments, but they can also be created effectively in straight-margined ponds by using the topography within the pond. There is no evidence to say that a well-designed, straight-sided pond cannot be as good for wildlife as a wavy-edged one.
Many older books recommend a minimum centre-pond depth of at least 60cm. However, the Freshwater Habitats Trust
points out that this advice originated from fish-keeping and that most invertebrate wildlife needs shallow rather than deep water. They suggest that the deepest area need not be greater than 30cm. In contrast, the National Amphibian Survey
concluded that amphibians (except frogs) were found less frequently in ponds less than 0.5m deep.
Deep ponds don't look very different aesthetically from shallower ponds, so appearance isn't a factor. One big problem with deeper ponds is that they are more likely to suffer with low summer oxygen levels at the bottom, especially if the pond gets a lot of leaf debris falling in suring the autumn. At worst the pond can go anoxic and smelly when disturbed, probably requiring a sludge pump.
So probably the most important advice is not to go for a very deep pond (and this will save a lot of digging) but to concentrate on its topography and make the bed of the pond to be as gently shelving as possible - steep-sided ponds are likely to offer the fewest opportunities for wildlife. If you can add undulations and irregularities in the bed, so much the better. Given that most biodiversity is in the vegetation at the margins, concentrating on this makes sense
Location: The most important decision regarding location is to make sure you will not encounter any services when digging (eg electricity cables, water, sewage and gas pipes). These are not often found in back gardens, but check your deeds for any information, and look to see where the services enter your house. Manhole covers are the give-away for sewerage. If in doubt you can hire a piece of kit called a Cable Avoidance Tool which can locate electric, telephone and metal gas or water pipes.
Shade: The other regular advice is not to place ponds in shade. Certainly heavy shade can inhibit amphibians, but equally other creatures including most amphibians prefer some shade. A mixture of sun and shade allows animals to move to cooler or warmer water as it suits them.
However, ponds directly under trees are prone to leaf litter falling in. So while 'Ponds should not be shaded by trees' is one of the pond myths identified by Biggs et al (1994)*, most gardeners are likely to opt for a sunnier pond not overhung with branches. Another good reason to do this is to avoid damaging the roots of established trees while digging the pond. If your pond will be overhung by trees, it is best to keep it shallow, or accumulated dead leaves in deeper water can decay and cause anoxic conditions limiting bottom life. The alternative is to net the surface off in autumn and try to catch the leaves before they fall in. Most of us forget to do this until too late.
*(New Approaches to the Management of Ponds. Biggs, J. Corfield, A., Walker, D., Whitfield, M. & Williams, P. In British Wildlife, 1994 Vol 5, 273-287)