Asm contracting

I provided a design drawing of what I wanted, and unfortunately the quote didn't estimate the materials correctly, so there were additional costs to get what I wanted. This is disappointing.
The project took longer than the contract specified, which is partly due to COVID delays but also due to inaccurate estimation of materials needed.
My advice is to be very very clear on customer requirements so that expectations are set from the beginning... and make sure there is no snow on the ground so that the contractor can see the space properly before signing the contract...