Here is the initial sketch of my design

Here are updated photos of our progress. My husband dug the trench and laid the pipe, and rocks.

We've dug out the basin area, amended with compost, and spread mulch.

The design and updated photos didn’t post. Wait until the grayed out photo gains full color before hitting submit. Often reducing the photo size a bit helps.

Looks like a good plan! Sounds like it will work fine. The size of the basin should be 20% of the roof area flowing to the rain garden. Or if you want to be conservative and you have clay soils, you could go 30%.

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