The pond should have a deep section, at least 45cm (18in.) deep (preferably 60cms (2ft) deep in colder areas) to give fish a good chance of surviving the winter. With this depth, even if the water freezes over on top, it should remain liquid in the depths.

The pond's sides should be carefully shaped to avoid a saucer-like cross-section. The problem with a saucer shape is that much of the water is very shallow. Steeper sides result in a greater volume of water in relation to the pond's surface area, which reduces the likelihood of temperature fluctuations.

Vertical sides to a pond are possible for a raised design, but more difficult for a sunken type, because the soil tends to collapse as you dig. Aim for as steep a sloping side as possible in this case, incorporating a planting shelf part-way down for marginal plants.

Make one portion of the edge more gently sloping than the rest so that creatures can enter and leave the pond easily. After all, it's quite common for hedgehogs to fall into ponds and drown because they can't scramble back out; frogs and toads find steep sides a struggle too.