Drainage is necessary if there are springs that cause wet spots in the garden; if water flows down from higher ground; or if, in winter, water lies in pools for more than one day after rain stops.
Plastic land drainage pipe has little slots
If there are springs, or water flowing from higher ground, a drain will be necessary to intercept the surplus.
The pipes, plastic land drainge or clay drainage tiles, should be laid to give a fall of about 30 centimetres in 30 metres. Make sure there is an outfall such as an existing stream or drainage ditch to drain the water into.
When water lies on a flat surface for long periods after rain without draining away, the problem is often caused by a layer of compacted soil below the surface. The passage of heavy machinery during building can compact the soil so much that water cannot escape to the subsoil below. Compaction often gets better with time but can take decades to clear, and sometimes does not clear.
Dig holes with a spade, or use a crowbar to make small holes through the compacted layer as deeply as possible. Fill the holes to nearly the surface with stone chippings. These holes should be about two metres apart if they are dug out with a spade, or 60 centimetres apart if made with a crow bar.
If this does not solve the problem, and the wet area is too small to warrant a pipe drain, or there is no outfall for drainage, put in a soak-hole. This can be made any size or depth, but to be effective it should be at least 60 centimetres square and 75 centimetres deep, or the equivalent. This is filled with fairly large stones or bricks or rubble, with finer gravel near the top and finished with about 25 to 30 cm of soil, or covered with paving, in which case the gravel can come to the surface almost. Short pipe drains can be run into the soak-hole to extend its drainage area. Digging out a hole of this size creates quite a lot of spoil to dispose of.
Remember that a wet spot can be used to grow bog plants, avoiding the need for drainage. if the wet spot is not in an important area.