EP, as well as local township ordinances that could be in play.If the tank is out of service because it is deficient, many local municipalities will require that first it be pumped, they filled with stone and the lid broken to render it non-functional, then graded over and planted like in grass.In each PA  municipality there is an official called the Sewage Enforcement Officer, SEO, contact the township get the name and phone number of the SEO and call him or her for a direct answer to your situation. 

Skills of value that I could offer would be architectural planning, design, 3d rendering, surveying, and of course hustle.

Once you know the relevant building restrictions you'll want to have an architect whip up some rendering and plans for what you can do in the space, finalize those, and then take it to the city (if you live in a city) for approval and permitting.Then comes the GC/Builder work -- prep the site, foundation, walls, roof, and then the rest you can guess.

This rendering looks great!

Has the OP paid him money for services rendered?

@Steven LoweCorrect me if I am wrong, but filing a lien requires means notifying the recorder's office (in the county or borough of the property) of failed payment for services rendered.

Lack of build-ability renders it worth nothing more than the parking lot it is being used for. 

first walk into your local planning department and public works and see if its zoned correctly and if there is infrastructure to handle that much sewer and water demand.. many times having to up size or do a lot of offsite work renders a project not fically feasible.

If a contractor or a neighbor or investor on another project, a contract purchaser, a partner, or the town, etc. etc. sues your LLC and it's a site-specific LLC (with few exceptions, of course fraud being one), as long as you maintain a separate bank account and hold each property's LLC out as its own entity, then if a judgment is rendered against you, it can only be collected against that single property, not all of them.

The early replacement of this stuff renders their schedule obsolete, but good luck getting them to recalculate the reserve account contribution.