oh dear no need for any worry if you get the client to ring up the suppler of the washing machine dish washer and ask for a hight reduction kit this is a flat pannel that takes the place of the origainl top about £50 to £80 have done this a few times its lower by 20 to 30 mm
And then send a bill to your kitchen fitters.
A good kitchen fitter will take into account levels, floor, appliances, floor coverings etc.If you get to the end of you install and wheel the washing machine in to find it won't go under your worktop, well i guarantee it will be the one and only time you don't consider your top height .
I've fitted a few hundred kitchens in my time and can honestly say i have never continued fitting once realising the floor runs out 30mm+