Minimum drop on washer drain?
International residential code says that a standpipe must be no less than 18&quot; above the trap weir.


International Residential Code 2012
Chapter 27 - Plumbing Fixtures
Section P2706 Waste Receptors
P2706.2 Standpipes.
Standpipes shall extend not less than of 18 inches (457 mm) but not greater than 42 inches (1067 mm) above the trap weir. Access shall be provided to standpipe traps and drains for rodding.

Check local codes, to determine what's acceptable in your area.