TOPIC: Septic Under A House

OK, so this is likely a bad question to ask, but what are the hazards of having the concrete septic vault located underneath a building?

Currently have a manufactured home (ie double-wide) that shares a septic system with two other units of equivalent size. The two units are approximately level with each other, while the third is lower. The three units are staggered in a row. The septic collection is under the middle unit, nearly completely located under the building. The access block is completely under the unit. A layout of the approximate setup is attached (crude as it may be).

What are the pitfalls to this setup? Is it significantly hazardous to have the vault under the building? Aside from the obvious smell of sewer gas (which isn't noted), would there be concerns for other gases that wouldn't be detected by odor and are there devices such as CO detectors that would be wise to put in place above this unit?

Since the lower unit is below the vault building, but not actually below grade of the vault, it has backed up multiple times including 2 months ago, so the vault was not recovered with soil, the cap was just replaced. We assume this is against Washington state building code to have the units placed as such, but not for sure.

Any input would be appreciated.

Location of the home over the septic tank is not ideal and would present several concerns, the primary being ease of service.

The septic system may not be appropriately sized for 3 homes especially if one of those homes has had a significant occupant number increase.  You are also now relying on all three unit occupants to properly use the system (not flush things which shouldn't be flushed, etc.)
The backups are likely a result of the drain field failing or the system stressed beyond original capacity.
Regardless, your local health department should have records and requirements for the system. Our county health departments regulate the septic system installs, inspections, and servicing here.  Various other states and localities will differ though. 
Primary hazard with a septic system is bacterial contamination.  Location of the tank below a building may present structural concerns for both the tank, its related systems, and the building/home itself. 

Not sure if that helps Dan.

Can't expound much more on what Bill said...those were all my thoughts as well.  Find the guy that "designed" the system and deliver a few choice words? 

I'm also with Bill, except to add that the location may be grandfathered in, or is permissable for MH due to their close proximity to each other. Regarding the location for a "regular" home, most jurisdictions require 5 feet from the foundation. I am unsure what regs are for MH's. I would make a call to your local Health Department or AHJ. 

Unless it's a really huge tank, along with a huge drain field, it is going to keep failing. The largest septic tank I could find was 2500 gallons when I installed one in 1997. I was told then that a typical family would use about 1000 gallons of water per month per person, which would cause the tank to continuously work, not letting the bacteria break down the solids before they entered the leach bed. They recommended my tank be pumped out every 24 months. At the time I had a family of four, 2 of which were teens, and I was told I would need to, as stated by the other guys, be careful about what goes in and how much water goes in the tank. My leach bed did fail after about 8 years. I would think that multiple homes on one tank is a recipe for disaster, and the pumping of the tank would be difficult with a house sitting over it. And,as stated before, the danger of bacteria would be a huge concern should the tank overflow, the nasty kind of bacteria you don't want to be any part of!

Thanks for the answers guys! Apparently the current installation is grandfathered in before the codes were changed to explicitly disallow this, even with manufactured housing. And yeah, when the septic guys had to pump it a few months ago, it was a tight fit for them to get into the tank, but thankfully the field is still in good shape. Will have to get some gas detectors just to be sure though.