Suds Relief Code Rules (obsolete with high efficiency front-load machines)
A note on section 711 IAPMO Plumbing Code Section 711-1: Suds Relief: Ever since the year 2000 all new washing machines and dishwashers come with front loading fixture doors which both require High Efficiency Detergents which produce no-suds. If they did the suds would become compressed inside the washing machine and dishwasher forcing the suds through the frontloading doors. This same section also lists kitchen sinks and bath tubs however they also use the new H.E. Detergents, but they could mandatorily use other soap items that could produce sudsing. This would also overflow the sink or bath tub. Very little suds would flow by gravity through the 1-1/2” size waste pipe to cause any real plumbing suds problems. Washing machines were the real problem with pumped water forcing the suds through a 2” waste pipe. Excessive labor & material costs for installing a Suds Relief Piping Systems in year 2020 Is no longer required. All the above sudsing information can be verified by all US manufacturers of detergents.
The original code reads: 711.0 Suds Relief. 711.1 General. Drainage connections shall not be made into a drainage piping system within 8 feet (2438 mm) of a vertical to horizontal change of direction of a stack containing suds-producing fixtures. Bathtubs, laundries, washing machine standpipes, kitchen sinks, and dishwashers shall be considered suds-producing fixtures. Where parallel vent stacks are required, they shall connect to the drainage stack at a point 8 feet (2438 mm) above the lowest point of the drainage stack. Exceptions: (1) Single-family residences (2) Stacks receiving the discharge from less than three stories of plumbing fixtures
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So, if you are in a single family home, single story home, and/or have high efficiency front-loaders, then you do not need to worry about suds relief in your plumbing.
Source of the code https://forms.iapmo.org/email_marketing/codespotlight/2017/July6.htm