All my rentals are detached houses. And I don't include washers and dryers... Since they are prone to break. They all have W/D hookups. I don't include elect/water in rent, so my issue is with getting a call, the W/D isn't working... The dishwasher is bad enough.
That said, there was a study that found the number one item people wanted in an apartment/house was private W/D. So if you don't have room to offer standard hookups, then you might consider including a stackable unit... or hookups for stackable units... which would be fine for people who are going to stay longer, harder for short term.
As with most "upgrades" return is hard to calculate... but generaly more amenities get the property rented sooner. And having a W/D might keep them in an extra year etc.
