Drainage under a Leamington terrace is usually older than anybody living above it and rarely laid out the way you would guess.
The spa era streets were drained as streets rather than as individual houses. A single run frequently serves several properties, passing under gardens and through walls that were built around it, and the manhole in your garden may be the only access point for a run serving four other households. That changes what can be built over, what can be diverted and who has to agree to it, and it is worth knowing before a design is settled rather than after.
Lightwell gullies are the second Leamington specific. They take water from a paved area below external ground level, which means when one blocks there is nowhere for the water to go but into the building. They are frequently the real cause of damp that gets blamed on the walls.
Where a run belongs to the water company rather than to you, and since 2011 a great many do, building over or near it needs consent granted before the concrete goes in.
On a system of this age we put a camera down before we design over anything. An hour with a survey turns a set of assumptions into a drawing. Our drainage work page covers surveys, repairs and new connections.