The Changing Landscape of Drainage Design

Traditionally, Architects might only show the direction of drainage on their drawings, often with notes such as “design by contractor.” Details like pipe sizes, invert levels, and manholes were usually missing. The problem was simple: contractors aren’t drainage designers. Without the right detail, they couldn’t price works accurately, and that often led to delays, last-minute changes, and problems on site.

It reminds me of a Flood Survey I undertook in 2020 where I witnessed this first-hand on at a recently built property. The house had zero-threshold bi-fold doors and was suffering water ingress. On the drawings, the architect had gone into detail about the roof pitch, downpipes, and guttering. But when it came to the ground, it just said “drainage by contractor.” The reality? The catchment and site hadn’t been modelled. A couple of channel drains were dropped in by the doors because that’s the “standard,” but there was no consideration of soil type, site gradients, topography, or connectivity. It could have been designed for anywhere, not that specific site. Sadly, this used to be very common.

Planning Has Changed

Today, the landscape has changed. Planning permission has far stricter requirements, with most developments needing at least a Surface Water Strategy to prove that water is being managed safely on site. That means more collaboration between Architects and Civil Engineers, and a much better outcome for the end client.

If your Architect says, “I can sort the drainage,” it’s worth asking: do they mean full design with pipe sizes, invert levels, manhole locations, and catchment modelling? Or is it just arrows on a drawing? Saving money early by skipping detailed design usually costs more later. Without proper plans, schemes can’t be accurately costed, connection agreements may be delayed, and what gets built on site may not perform as it should.

Why It Matters to Get It Right

By involving a Civil Engineer early, you unlock the site’s full potential. Costs can be considered from the start, with value engineering built in to save money, not spend it.

At FPS Environmental, we design everything drainage-related, from SuDS Strategies for planning, to detailed foul and surface water designs, Section 106 connection agreements, and Section 104 adoption agreements. We also handle pre-planning applications to check if the local sewer network can accept your site flows, so you know upfront if upgrades will be needed.

We’re often instructed to assess and improve drainage at existing properties, or to review high-value infrastructure and buildings across the country.

With the changing landscape of drainage design, getting it right from the start saves time, money, and stress.

Written by Simon Crowther BEng (Hons) FCIWEM C.WEM MIET

23rd September 2025