Drainage is where Daventry extensions go wrong quietly, and where the cost of getting it wrong turns up years later during a sale.
The first question on any rear extension is whether a public sewer runs under the footprint. Since 2011 a great many drains householders assumed were their own have belonged to the water company. Building over or close to one needs a build over agreement, applied for and granted before the concrete goes in. It is a routine application and a serious problem to be caught without, because it surfaces during a sale when you are least able to deal with it.
Estate housing has a particular pattern worth knowing. When a large area is built at once the drainage is laid out to a scheme rather than house by house, which means runs frequently cross plot boundaries in ways that are not obvious from the manhole covers. On a Daventry estate we establish what serves what before we dig rather than assuming the drain in your garden is only yours.
Surface water is the other half. A soakaway only works if the ground will take the water, and that is a site by site answer rather than an assumption. A percolation test costs very little next to a soakaway that fills in October and stays full until spring.
Our drainage work page covers surveys, repairs and new connections.