You need to tell your client that the term "flatwork" should not be taken literally. OK, bad joke.
Seriously, though, I agree wholeheartedly with TGLG in that codes are written to (among other things) provide some factor of safety in construction. We don't live in a perfect world and aside from the fact that stuff almost never gets built exactly right, also consider that the materials (both manmade and natural, such as soil) involved in construction may not always behave exactly as predicted, either.
More to the point here, there is always the possibility of some future settlement of the concrete flatwork around the building. It could be due to water infiltration (maybe through cracks or seams in the concrete?) or for other reasons. If the flatwork is already as flat as 1% away from the building, even some minor settling could result in parts of it ending up "dead flat" with no drainage away from the building, or (even worse) draining towards the building(!).
So even if you had the best contractor in the world and built the concrete exactly at a 1% slope today, consider that future settlement could change that. A 2% slope on the concrete will allow for some settlement and still provide for positive drainage away from the structure.
Bottom line...TGLG is right. Talk to your client.