Under NO circumstance should you 'tap in' to an existing building sewer line that is sharde by 'two neighorbors'.

NEVER!

EVER!

You should ONLY install a building sewer under and and all circumstances that services ONLY YOUR indiviudal single family dwelling unit. Period.!

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.

OK, thanks for the info. I think we use the BOCA Code but it's probably similar to one of the others.

I'd like to get an official easement and contract outlining maintenance, but such a thing doesn't exist right now between the two homeowners who share the pipe. I'm worried that too much legalese will give them cold feet with granting permission. But it really is in everyone's best interest. I'll test the waters and see what happens...

The term BOCA stands for "Building Officials & Code Authorities" which was an agency that stood for 193 years creating building codes in the USA. In approximately the year 2000 BOCA was renamed the ICC (International Code Conference).

When they changed the name of the agency from BOCA to ICC the BOCA Plumbing Code was renamed the IPC (International Plumbing Code).

The IPC is a broad code that covers all structures, both residential & commercial and can be very complex to figure out. For simplicity they then developed the IRC (International Residential Code).

The IRC governs single family residential dwellings & residential multi-family dwellings where the structure does not exceed 3 stories in height or have more than 4 individual dwelling units.

Any multi-family dwelling structure that exceeds 3 stories in height or has more than 4 individual dwelling units is defined as a "Commercial Residential Dwelling" and all the plumbing must be installed in accordance with the IPC commercial plumbing rules.

Taking this to the next step, in jurisdictions where they have a distinction between Residential & Commercial Plumbers a Residential Plumber may only work on single family or residential multi-family dwellings, whereas a Commercial Plumber may work on all structures.