AFAIK: If you want to claim cheaper rate for part of it, you need to install a second certified meter for that part. If you are willing to go by default rate (which at least in some places assumes some ratio) you don't.
Ah. I guess this is all because you don’t get charged for sewerage on the lawn watering.
That's how it's done everywhere I know of in the US. You do have to pay a bit more to have the second meter, but since sewage treatment is often more expensive than initial water treatment, you save quite a lot on balance.
Can confirm, I’ve seen a dual-meter setup for garden water in Germany.