What kind of professional do I call first?

[stinkingbishop]

This is probably a really daft question.

Background: moved in 10 years ago, huge garage at the bottom of the garden, built without proper drainage on a concrete shelf, shoddy roof, so over time repeated floods, and now roof coming in. We want to tear it down and put up a new building - part garage/storage, part gym, part teen hangout. Issue is I don't think it's just the poor build that's causing the drainage issue - that whole part of the garden floods, doesn't seem to soak away, and it's downhill from the front of the house where the mains is (or whatever it's called, sorry, basically where our house drains connect to the sewer) so it's not as simple as connecting it to that.

I think, rather than getting an architect and going straight to fancy drawings, we need someone who understands land and water and drainage and what is and isn't possible. So, who is that? A land surveyor? Structural engineer? Someone else? Would a decent building company do that/have access to the right people? Or would it be ok just to go to a decent architect and they could bring in the right people?

Issue at the moment is I don't even know the search terms!

I think you're right, I don't think an architect is the right person although they may have good contacts and can usually offer the project manage the whole build including getting the right engineering input and appointing contractors, but that usually costs £££ to get them to do start to finish. If it's otherwise a straightforward project you probably just want the architect to do the drawings and design part and take those to your structural engineer and builder separately.

I think it's potentially a structural engineer you want, or there are specialist drainage engineers as well. When we had drainage and sewer problems on our extension we actually started off with a very good builder who (a) was the one who put his finger on what the problem was (b) eventually was the one who came up with a practical solution to what we needed to do, but in the middle the involvement of the structural engineer was pretty crucial as well as proper architectural/technical drawings which is what the structural engineer works off.

Thanks @maxelly. That's very helpful. So, just to be clear, you'd get architect in to do drawing first, then either get structural/drainage engineer, or go straight to builders who have links with engineer?

Get a builder round to look at the site and tell you what needs to happen. It's very specific to the building and location

A flood and drainage engineer is who you call if you want to sort the flood issue. Or get a modular building like this one and ask it to be raised off the ground so you don'thave to worry about flooding. They have ground screws that can raise it.
https://www.tigermodular.com/garden-rooms/
They do all the drawings too. No need for an architect.

"...we need someone who understands land and water and drainage and what is and isn't possible. So, who is that? "

If you google "garden drainage specialist" then you will get a number of different companies with lots of experience in this area.

With the garden sloping away from the house it sounds tricky but I'm sure that this situation has come up before.

Thames Water came and inspected ground water and flooding in our house before last.

Call the National Flood Forum. They provide advice to flooding of properties - nomally houses but they will know someone in your area who could help.
The lady who runs it, Mary, is super helpful.
Defo worth speaking to the water company too though. They could do a drainage survey - you may have a cracked pipe on or near your property causing the excess water.

Project manager first who will then know the right people and order to do it. So appoint the construction team first would be my suggestion