Poured slab on grade concrete foundations, as a minimum, consist of: 1) A reinforced concrete footing which extends below your freeze line in the ground; 2) A reinforced slab on grade poured over an under slab moisture barrier and insulation. The use of moisture barriers, insulation, fill and reinforcing materials will vary with the conditions of your site and design. The size of your footings will depend on the load they are designed to carry. For example, a one-story structure will have smaller footings than a two- or three-story structure. Your local building official can help you with specifics based on your location and the type of construction you envision. A blocked sewer line under a slab on grade can be repaired by removing the slab in the area of the blockage, excavating and replacing the piping where needed. It is not an unusual situation in remodeling of older houses, but it is messy and time-consuming. Good luck!

Normally when connecting single family residential structures to a municipal sewer system they require each structure to have a separate line, however you stated that both your contractor and the town have suggested combining the lines so it goes without saying that you will be granted an exception to the individual line rule. The question then becomes, Can the single 4" line adequately handle the load of all three structures? It is impossible to give an exact answer because you did not mention where you live and without that information we cannot determine if you are under the IRC(International Residential Code) or the UPC(Uniform Plumbing code"). When designing a DWV(Drain,Waste & Vent) system we begin by listing all the fixtures in the structure that will ultimately be connected to the drainage system. We then refer to a table in the code that assigns a DFU (Drainage Fixture Unit) load value to each fixture. In addition, for some fixtures the code assigns a minimum line size. I.E. a shower or laundry standpipe is required a 2"line and a water closet is required a 3" line. At any given point within the structure where two or more lines connect we must compute the DFU load at that point and insure the line downstream from that point is large enough to handle the total combined DFU load. Te determine the size of the "Main Drain" and "House Sewer" we compute the total DFU load for the entire structure then consult another table that lists a maximum number of DFU's that is permitted based upon size the pitch and size of the line. To provide an example I have worked up your problem based upon the IRC tables. (Even though the actual DFU values and line load maximums are slightly different in the UPC, the end result would be approximately the same.) Obviously we would need to know exactly what fixture are installed in each structure but for the purpose of illustration we sill assume each bathroom to be a standard bathroom group, Lavatory, Tub/shower & a 1.6gpf Water Closet. Lavatory............... 1DFU Tub/shwr............... 2DFU Water Closet(1.6gpf)...3 DFU Bathroom Group total = 6DFU's Four bathrooms = (4 x 6DFU's) = 24DFU's Kitchen Sink with Dishwasher....2DFU's Laundry Standpipe.............. 2DFU's Total per structure equals ... 28DFU's The existing load on the 4" sewer line is two structures so the existing load is 28DFU's x 2= 56DFU's. If we then add your structure the new combined load would be 28DFU's x 3 = 84DFU's. A 4" House Sewer with a 1/8" per foot pitch is rated for a maximum of 180DFU's and if the line has a 1/4" per foot pitch the maximum is 216DFU's therefore even with your house tied in, you will still be less than 1/2 the maximum allowable load for a 4" line. Now in regards to access for maintenance and the respective cost to each homeowner. Normally the municipal sewer provider is responsible for maintaining the last 3 to 4' and the tap into the municipal sewer line. Each homeowner is then responsible for all remaining line to the structure. In the case of a shared line such as you suggest there should be a contractural aggeement between all parties that the shared portion of the line should be treated as a common main and the maintenance cost should be equally shared by all three homeowners. Each respective homeowner should then be responsible for the maintenance cost of the line from their structure to the tie in point on the shared line. The contract should provide each respective homeowner an easement to allow access for maintenance of their line to the tie in point on the condition that they will be responsible for both the cost of the maintenance and any landscape that is disturbed during the maintenance process.

Pursuing the logic of the above, using "vent blockage" as a core problem, it will manifest itself as a sometimes-open drain letting sewer air into the house. One of the drain holes is letting sewer air into that bathroom at night. A drain has a P trap to prevent this. Thus, one of the P traps is being "siphoned" or emptied. Tape shut one of the drains. Use masking tape. See if that one is the one letting sewer gas in at night. Try again with another drain. Repeat. If the only drain hole is the bathroom sink, you only have one drain to stopper, to try this. The seal must be perfect. Use tape. In the morning when you take the tape off, the P trap will still have its water in it so you will not smell gas there even if you try real hard. Once you have done this testing, it will confirm to you what the problem really IS. This is valuable knowledge. It enables you and your plumber to know clearly what to do next. Report your findings and await further instructions.

I've never heard of such. I believe city engineer doesn't know what he's doing so I suggest you find out the facts for yourself. Most states have water districts that govern ground water and waste water,talk to them before installing this. I would at minium want written approval from city to protect myself in case there's a problem in the future. I don't see how it could negatively effect your plumbing.