Hybrid Hot water heater is tall!

I bought a hybrid hot water heater to replace my electric heater. It's a lot taller than my current heater and may not have enough clearance. Manual says give it 6" to have room to clean filter on top (It's flexible, so I can still get it out). It will probably be a little short of that, but it's not a solid ceiling above it, so there is some wiggle room. Will this not pass inspection in this spot without a certain clearance? I could only find mention of 18" online, but my current hot water heater doesn't have that much clearance. I've already figured that I need to install with side connections to allow filter access. Also, I don't currently have an expansion tank, would there be any need to add one? Any advice is much appreciated.