Q: Due to roof run-off meeting a slope going into the house, leading to spongy turf and damp conditions in the floor of an added sun porch, I plan to install a French drain about 12' from my house. I presume the drain should be below the elevation at the house, but how much, the drain has to go about 20' across the back, then about 40' along the side to the street.--(it's a lot of digging)?
That's beyond my area of expertise Bruce. I suggest you talk to your city contractor or pay for a consult from some who has expertise in this area. Water is tricky and it can cause a lot of damage, so don't guess. Best And Happy Yardening, Nancy