The size of pond is only limited by the size of your garden and your budget, but do go for the largest one you can. This is because the temperature of large bodies of water doesn't fluctuate as much as smaller volumes. Swings in temperature affect fish badly and sudden warming encourages green algal growth too. As a rough guide, aim for a minimum of 4 square metres (40 sq.ft.) surface area, i.e. 1.3m (4ft. 3in.) diameter circle or 2m (6ft. 6in.) square.

Don't rule out a smaller pond though, but be prepared to give it plenty of loving care and attention. You'll certainly need to install a pump and filter to keep the water clear.

Garden designers talk about formal and informal shapes, but what do they mean? Well, formal shapes are those you drew in geometry class at school (squares, rectangles, circles and ovals) and look good as raised or sunken ponds. Informal ponds have irregular and more natural outlines, and are easier to create as sunken rather than raised ponds .

Formal and informal ponds look good in a formal garden layout, but a formal pond in an informal garden does look odd.

Once you've decided on a site, size and shape, mark out the pond's outline with a hosepipe or canes, then give yourself a week or so to ponder before beginning. Move the outline around if necessary and only start digging (or building) once you're completely satisfied that you've got it all right.