Plumbing For Washing Machine, Is This Right?

[BillyBB]

Just moved into a house and connecting our washing machine where theirs was. Turns out to be a bodged diy job.
The outlet runs horizontally against a slight rise into a drain-out kit with one way valve. However this is fitted after the sink trap.
I thought these had to tee in above the trap.
Is the one way valve an acceptable alternative to a trap please?

[The Builder]

Modern traps do have a spigot above the sink trap, to attach a machine hose.
However .... what you have, although rather convoluted, is fine, as long as the non-return valve works Ok.