In Britain the Building Regulations impose certain restrictions on waste and drain pipes. Drains must have a sufficient slope to enable the water to run away quickly.
Changes in the direction of a drain have to be fitted with an inspection chamber, and connections to existing drains are best made at such points.
Waste pipes in houses built in Britain since 1966, sometimes have to be inside, and this also applies to additional pipes. Additions to houses built before 1966 , on occasions, can be fixed outside along with the original pipes. Each waste pipe requires an ‘S’ bend water sealed trap to prevent drain odours from reaching the appliances. However, pipes from different sources (e.g. a sink and washing machine) can meet inside the building so that you only have to penetrate the wall in one place.