Main reason is because you want one! Benefits are: its very soothing to look at and interesting to see all the wildlife it attracts - frogs, dragonflies, newts if you're lucky and much more. You need to decide whether you want a natural wildlife pond or a formal one - I reckon formal ones are easier to build, but if you go for that option make sure there is somewhere that tiny froglets etc can climb out - a basket of eg water iris in one corner would do nicely.
Down side is occasionally you may need to empty it, especially if it isn't big enough - you really need a minimum of about 8 x6 feet for the water to stay balanced well. You will need to put a net over it in autumn if there are trees nearby, to prevent a lot of leaves blowing into it. Digging the hole is hard work and you need to put the excavated stuff somewhere.
Get some books from the library about garden ponds - you will find heaps of helpful information. Better than looking on the net as you can keep referring to them.
Before you start
It's easy to create a simple pond in your own garden. Always consider the following: •The view of the feature from every angle of the garden and house. •The type of pond - will you use a liner or a preformed pool? •The size of the pond. •The type of plants you wish to grow. Different plants require different depths of water. •Avoid shady areas, especially near deciduous trees. •Seek professional advice when constructing complicated designs and using electricity to operate pumps.
What to do
Marking out •Define the perimeter of the pond, either with string, a length of hosepipe, or by trickling a layer of dry sand through your fingers. • Remove the turf from the area and stack neatly away from the working place.
Digging •Excavate the area with a spade to the depth of the first shelf. Mark the outline of the shelf with sand or string. •Dig out the centre of the pond and either add additional shelves, or dig until the required maximum depth has been reached. •Check each shelf is level.
Wildlife in mind •Move the dug-out soil to another area of the garden. •Check the sides of the pond following the profile you intended. It's a good idea to give one side of the pond a gentle slope to offer animals, such as hedgehogs, a means of escape if they fall in.
Lining the pond •Remove any lumps or sharp stones from the hole. Then spread a 3cm (1.2in) layer of soft builder's sand over the area. This will help protect the butyl liner being punctured. •A layer of pond underlay or old carpet above the sand will provide additional protection. •Get a friend to assist in unfolding the butyl liner. Spread it evenly over the hole taking care not to damage it by dragging it on the ground. Secure the sides of the liner with bricks.
Filling up •Start filling the pool with water and pull the edges of the liner so that it fits neatly over the contours of the pond. •Continue filling the hole with water until the pond is full. •Trim the sides of the liner leaving a 30cm (12in) overlap around the sides of the pond. •Cover these with paving slabs, or if you want to create a more natural effect, lay turf up to the water's edge. •The liner should be covered to prevent sunlight causing it to perish. Position plants and add oxygenators. If you want to add fish wait six weeks until the plants are established.