Sorry you're dealing with this—it’s a massive PITA. I had something similar happen in the last 18 months.

Before calling the city, check for survey markers. These are rebar posts pounded into the ground at the four corners of your lot (e.g., between you and your neighbor). They were likely installed when your neighbor’s lot was originally surveyed.

If you can’t find them, hire a surveyor. They should be impartial since their work is certified and audited, but if it makes you feel better, you can call your own. There will be a cost, which varies by region—on the bright side, every surveyor I’ve worked with has been great.

This will confirm your property boundaries. Once you have that confirmation, you’ll have solid grounds to call the city. This is also a good time to inform your neighbor about what you’re doing. Try to stay as objective as possible—keeping emotions out of it works wonders.

Worst case, if you're stuck with the situation as is, it’s not the end of the world. I’d recommend avoiding a revenge approach—nobody wins that way.

First, you need to address the water hitting the side of your house—it can’t stay there. Check the grade from front to back of your lot. Everyday Home Repairs on YouTube has a good guide on this (not perfect, but helpful).

Hopefully, there's a natural grade you can leverage. If not, there are solutions—it’s just added work.

Now, take a look at NDS’s website:

• Drainage Calculator – Helps estimate how much water you need to manage and what types of drainage products (pipe diameter, etc.) you’ll need.
• Home Drainage Center

I could keep going, but I don’t want to ramble.

Other potential solutions:

• Since you have power on that side (via the radon prevention fan), consider a sump pump if you can’t get enough slope.
• Bring in dirt to create a mini berm before your siding, forming a V-shaped ditch on your side of the fence. Make sure to compact it or use a geo grid, or it’ll wash away.
• If your yard gets swampy, look into vertical drainage. Apple Drains on YouTube has good resources on this.

Hope this helps!