Sewage ejector to service detached accessory dwelling -- location of basin?

[AaronC]

Hello,
I'm working on plans to build a detached accessory dwelling unit behind my house. Located in Whatcom County, Washing state.

The location of the detached accessory dwelling unit is approximately 30 feet back from the main house. The grade slopes down towards the accessory dwelling unit, loosing approximately 12 inches over the 30 feet. This leaves insufficient slope for accessory dwelling waste drain line to be gravity fed to the main house where it would tie into the main sewer line in the crawl space that runs to the street.

My question is: If I were to utilize a basin with sewage ejector pump, where is the appropriate location for the basin/pump? Should it be located in the crawl space of the main house, low enough for the accessory dwelling to feed it via gravity and it only pumps the elevation to reach main sewer line OR should it be in the accessory dwelling unit where it pumps the horizontal run + elevation gain to reach the main house main sewer line?

The attached picture is a crude illustration of the first scenario. Any advice is appreciated.

 

[Smooky]

The grade of a sewer line under pressure doesn't matter so I would put the basin just outside of the accessory dwelling unit. It would be easier that way.