Code requires that a washing machine must discharge into an approved indirect waste receptor. An approved waste receptor is either a washing machine standpipe or a sink.

Prior to the mid 80's both a kitchen sink and a washing machine standpipe were 1-1/2" pipe, however in the 80's they changed the code where a washing machine standpipe is now required to be 2", but if the standpipe was installed before the code change an 1-1/2" line still meets code.

Now in regards to the washer standpipe being connected to the kitchen sink drain, not only is that fully code compliant, there is nothing in the code that would prevent you from discharging the washer directly into the kitchen sink, in fact. some small portable washing machines for apartment dwellers use the same type of hook up as a portable dishwasher, simply connecting to the kitchen sink faucet.

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