Sloping garage floor

If you use a hose in your garage, you're going to get water where it shouldn't be.

If you just need to get random crap out it's better to use an electrical blower for that, and if you need to wash the floor - use a mop and a bucket, like you would in the rest of the house.

PS: Also having a sloped floor in the garage is a safety hazard. If you forget to engage parking brake and leave the car in neutral, it will roll, potentially crushing someone.

flat floor but don’t put a 5-10cm step to the house, either flat or a proper step, in between is a trip hazard. Have a rebate for the garage door too.

You don't need any steps at all, unless your block requires a split level house. I'd leave designer to draw pretty pictures, and let the builder do his job. Garage is a pretty standard thing, you just need to tell the builder what dimensions it should be, and how tall you want the gate to be (I'd go as tall as the ceilings allow)

I would always have a step into your house, and a slight slope 10mm per 1000mm max (maybe 50mm overall) towards the garage door is also not the worst idea. That is the exact set up I have in my garage.

A car won't roll on a gradient that small and no one would know that the floor even slopes without putting a level on it, but just incase you ever had a large volume of water for what ever reason in the your garage it will make sure that it doesn't breach the main house and is encouraged to head back out of the garage door rather than just pooling in there. Your external concrete driveway should also sit slightly lower than the garage floor it abutts to prevent wind driven water from being blown back into the garage.

A step at the internal garage entrance isn't an issue as that door is the same as any entrance or exit from the house that typically involves a step down for exactly the same reason.

When you say slope, most people think of a discernable angle that you can feel.
No one could stand in my garage and tell that the floor has a very slight gradient. A car won't roll on it even when the car pulls into the garage wet and water runs off it it's not like it rolls out the garage door, but if I dropped a hose in my garage the water will slowly head to the garage door and into the cutoff drain rather than towards the internal house door. It's just a small added protection in a worse case situation that will probably never happen but I'll be glad I did it if it ever did.

It's similar to the floor in your bathroom. It gently grades to a floor waste instead of out the bathroom door. You don't walk into your bathroom and necessarily notice the slope because it is very gentle. Most builders just put in the garage level and particularly in the eastern states where your garage is part of the main slab it makes it easier for the builder if it is level.

Thank you all for your advice and inputs. We have decided to add a step and a 5 cm fall form the middle of the garage to the draft plans. We'll see how we go with the builder(s)