I have two layers, parcel and polygon. When I create a new building alongside a parcel it produces a gap between the parcel and building along the side.
How can I clean this gap or how to avoid it?
You can use the Integrate tool to clean this gap:
Integrate is used to maintain the integrity of shared feature boundaries by making features coincident if they fall within the specified x,y tolerance. Features that fall within the specified x,y tolerance are considered identical or coincident.