I am inspecting a house that has a 4" drain line that reduces to a 3" drain line, is this acceptable?
3" is OK for toilets in my area.
If I remember correctly, a 3" waste line can handle 3 toilets and a 4" to 3" reduction can take place in a vertical line under certain conditions…but, it’s been a while. 
I do not know of any time a line is allowed to be reduced.
I don’t see a problem with a 3" toilet drain.
As far as I’m concerned you can increase size but never decrease size.
The ultimate question is how did it work?!
As Larry pointed out, vertical applications may work (if in fact there is enough head pressure above the reducer).
An after construction inspection is pretty easy to figure out. Seeing it passed code inspection, if it’s working why are you going to discuss it?
Write up the reduction location and advise the client that if they have problems with sanitary drainage to look in this area. Right now it works…
Home inspectors are not designers or code enforcers.
Save yourself an argument for something important.
Source please.
Common sense as it will cause blockage.
Must always increase and never decrease.
Same rule apply s to vents.
P3002.3.1 Drainage. Drainage fittings shall have a smooth interior
waterway of the same diameter as the piping served. All fittings
shall conform to the type of pipe used. Drainage fittings
shall have no ledges, shoulders or reductions which can retard or
obstruct drainage flow in the piping. Threaded drainage pipe fittings
shall be of the recessed drainage type, black or galvanized.
Drainage fittings shall be designed to maintain one-fourth unit
vertical in 12 units horizontal (2-percent slope) grade.
Commentary: So as not to retard the flow in drainage piping, drainage
fittings must have a smooth interior water way. Fittings
must be free of ledges, shoulders or reductions
that can retard or obstruct the flow within. Recessed
threads are required in drainage fittings so that the interior
bore remains constant in the transition through
the pipe and fitting, thus maintaining a smooth water
way. Drainage fittings must also be manufactured
such that a horizontal pipe connection to the fitting will
naturally be sloped at one-quarter unit vertical in 12
units horizontal (2-percent slope).
P3005.1.4 Water closet connection between flange and
pipe. One-quarter bends 3 inches (76 mm) in diameter shall be
acceptable for water closet or similar connections, provided a
4-inch by 3-inch (102 mm by 76 mm) flange is installed to
receive the closet fixture horn. Alternately, a 4-inch by 3-inch
(102 mm by 76 mm) elbow shall be acceptable with a 4-inch
(102 mm) flange.
Commentary: The reduction from 4 inches (102 mm) to 3 inches (76
mm) by means of a 4-inch by 3-inch (102 mm by 76
mm) closet flange or a 4-inch by 3-inch (102 mm by 76
mm) closet elbow is acceptable and is not to be considered
an obstruction, as would be the case with reducing
drain sizes downstream of the connection. A 4-
inch by 3-inch (102 mm by 76 mm) closet flange is referred
to as a reducing flange or simply a 3-inch (76
mm) closet flange.
The flange is always 4 inches and is not the actual drain run.
The flange is 7 inches in diameter…blah blah.
Drain pipe shall never be reduced in direction of flow at any point .
The drain elbow can be reduced beneath the flange. That’s what it says. See it all the time. Now, if it’s downstream, I agree no reduction would be allowed by code.
Since we have no pic we don’t know were the reducer was.
There is no problem with going from a 4" toilet flange to a 3" line.
But that is a guess to what is going on here.
Waiting for more info.
Joe…read the question.
I am inspecting a house that has a 4" drain line that reduces to a 3" drain line, is this acceptable?
The flange is almost using the actual Bathroom sink basin opening as a example of reduction.
Correct it can not be reduced at any point during a run unless you feel like rodding out the line after each use.
Without seeing the picture I have no good comment. Smooth sides and 1/4 per ft down is critical. Where the transition is from 4 to 3 may not work to protect from buildup inside the walls. Even the type of toilet can make an effect on proper waste removal.
I totally agree