Great question — answering as the UphillTech team:
15x15" is too small. You will need to enlarge it not only to allow for your 18" diameter sump, but also to to allow for 2-4" layer of drain rock surrounding it (that is, plan for a 24" hole.) The circle you've drawn seems about right. An electric chisel (you can rent these) will make this job easier. By the way, the sump basin should be the kind that already has holes in it (like this one ), vs. a sewage basin which will not have holes. Check to make sure. Since this sump already drains into storm discharge I don't think you need any permission from your city to install the sump pump, but check before you start digging. You will need the pump to be on a GFCI circuit to meet code. There used...
Full answer by hacktastical: https://diy.stackexchange.com/a/315800 (CC BY-SA 4.0).
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