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Self Build Drainage
Laying the drains is something that can be achieved by the self builder. You will need a small digger which you can hire from the many hire companies around, and there are plenty of small companies (often one man companies) that will hire a digger and driver for less than the hire charge for a digger from a hire point. If you take this option, it is likely that the contractor may have experience of laying drains and may be prepared to help as well as excavating and back filling.
Work out what materials you need in advance, but be prepared in that the layout may vary from expectations once the trenches are dug and you determine how everything will actually fit together. The position of 4" soil and ventilation pipes, rainwater down pipes and 4" underground pipes may be shown on architects plans - but make sure that these are sited taking into account the actual positions of sanitary ware - where you may site them in different positions to those shown by the architect.
When the drainage pipes have been laid in the ground they will need to be covered with pea gravel and inspected by the local authority. If you are connecting into a main sewer the local authority will also need to be in attendance.
Off Mains Drainage Systems
There are a number of companies catering to this market and the products available are surprisingly diverse, from a relatively simple septic tank with an overflow, to electrically driven treatment plants of various descriptions, right through to natural reed beds, which can be a great feature if you have enough garden space. Look to pay between £3,000 and £8,000 to provide off-mains drainage.
Remember that each site is different. Some sites have planning conditions attached which specify what sort of drainage should be used. If not, then your building inspector will advise as to your choices. Generally, septic tanks are the simplest and cheapest option, but you may not be allowed to let the liquids run off into the ground.
