I don't think you have to design it for a 24 hr storm. Your house has survived 90 yrs with only surface drainage.
If you saturate the drywell, the excess can be released to the surface were you deem approraite and grade permits.
It will be interesting to hear what the quotes end up at and how you move for forward. Maybe the high guy will give you a discount if you provide some on-site munchies.
Good luck
It has survived, but barely. The sill plate is currently rotting in a couple places. Left as it is, it's dying a slow death. There's been some wood replacement in the past due to rotting, and poor drainage is a likely cause.
Also, when you pour a concrete slab walkway that raises grade ABOVE the level of the sill plate, bad things start to happen. When that slab is removed (which I will be doing in a month or 2), and things graded to keep dirt below the sill plate again, the rest of the yard will be higher, creating a moat situation. The 2 1/2 inches of rain we got a last weekend made that evident. We have removed some brick pavers that were set on top of an old concrete walkway, and water just pooled next to the house. 3" of water right against the foundation.
Also, all of the gutters were replaced right before we bought the house, so some of them now drain in VERY poor locations. To drain them far enough away from the house would be tough to do with the location of my house in relation to property lines.
And while the house has survived 90 years, the garage has not. The garage has been rotting for years. It's too low, and floods frequently. On one side, the sill plate is GONE, and the studs are now rotting. The garage is essentially sinking. The garage is racking, and one of the walls has shifted enough to fall off the slab. A new garage on top of fill to raise it higher will encourage more water to flow to the lower "moat." I can't put a new garage in a location farther from the house due to zoning restrictions. As it is, I will need a variance to even replace it.
My neighbor's garage is 3 feet behind my house, and water just pours off the back of the roof to the ground next to my house (no gutters on their garage.)
And, the surface drainage that does get away from the house sometimes floods the back yard.
There's not really a place that grade permits to release overflow from a drywell. Only good place is right near the sidewalk, and I can't do that legally.
So, it's not a simple, easy, or at all normal or common situation. I've mulled over my options for the last 2 years, and played with different gutter setups, but we need to do something. And it's either a large drywell, or connect to the storm sewer.
On that note, I talked to an estimator for one of the companies giving me a quote. He stated that in the city I'm in, the stub from the main line to the property line is usually sloped to be at a depth of 4-5 feet at the property line, so I may actually be able to hand dig that. 4-5 feet seems a LOT more feasible than 10. One other plumber told me 5 feet is a normal depth, so I'm believing what I hear from people who dig these up as opposed to someone in the city govt making a guess based on where the main is under the street. And 5 feet makes more sense. My understanding is that the storm line is often kept ABOVE the sanitary line, and 10 feet down would be below it.
He hasn't formally completed the written estimate, but his ballpark after seeing the job was about $1500 to bed and lay 85 feet of SCH 35 pipe, supply install a 20X24 catch basin, connect 1 of the downspouts, and pull the appropriate permits and get inspections. This is with me providing a trench, a hole to the stub, and providing gravel to bed the pipe in.
$1500 is a deal to me, as it's close to what I was going to spend to do the drywell system anyway.
So, I'll wait for a formal estimate from him, and waiting on estimates from the other 2 plumbers that looked at it.