Attic bathroom help...

[saleenmav]

I was hoping that someone here may be able to give me a little advice. We are building a bonus room over our garage and are now contemplating adding a bathroom. My question lies in the drainage for the bathroom. The home is located in Effingham County, GA and they use 2006 IRC plumbing code. This is a single story home built in 2004 on a slab with a septic system. In my attic, about 10 feet from the proposed bathroom is the main vent from downstairs. It is a 4" pipe that leads down to the laundry room where the washing machine drains into it, then down into the slab attaching to the main septic line. The vent pipe in the attic also has a 2" line attaching to it just before it goes through the roof that appears to be coming from the master bath below. My question is, can I use the main vent as a wet vent(?) and drain the new bathroom into the vent where it comes up into the attic. I can easily vent the new bathroom separately as well, if more venting is needed due to the lateral distance from the existing vent. The other alternative I can do is remove the sheetrock in the laundry room and run a 4" pipe down alongside the existing main vent and tie into it just before it enters the slab, this way I'll be draining at about the same level as the existing first floor fixtures and pretty close to directly into the main septic line. The septic system exits the slab on the opposite corner of the house so there really isn't any other way I can see getting the new bathroom to drain into the main drain line. Do any of these solutions seem like they would work, and would they be safe and within code? Thanks in advance for any comments or alternative recommendations.