Why drainage should come before the layout
On a development site the levels, the drainage strategy, the attenuation and the outfall are one design. Where the water can go determines what floor levels can be, which determines how the site can be laid out - not the other way round.
The same logic applies to flood risk. Where a flood risk assessment and a drainage strategy are both required, the same team should write both, because two consultants producing two documents that do not quite agree is a reliable way to generate a planning query and lose a month.
The Yorkshire flood picture
Flood risk here is recent memory rather than theory. The June 2007 events across South and West Yorkshire, the York floods of 2000 and 2015, and the 2019 flooding across the region are all inside most clients' experience.
It also comes from more than one direction, and they get conflated: tidal risk on the Humber, fluvial risk from the rivers, and surface water flooding - which on flat, pumped-drainage ground is frequently the dominant mechanism and the one people plan for least.
On domestic work
The recurring domestic questions are simpler but no less consequential: what fall a run of drainage actually needs, where a new extension's drainage can connect, what happens when you build over an existing drain, and whether a soakaway will work in the ground you have.
Our drainage falls calculator handles the first of those directly, and the guides here cover the rest. Where a build-over agreement or a diversion is needed, that is worth establishing at design stage rather than when the groundworker finds the pipe.
What we do on this side
Drainage design and drainage calculations, SuDS design, flood risk assessments, Section 104 and Section 278 submissions, levels and earthworks, and highways design - alongside the structural work, in the same practice.
That combination is the point on a sloping or constrained site, where the levels, the drainage and the foundations are one problem rather than three separate consultants.