It is a good idea to build and prepare your own surfaces to ensure quality. Any artwork large scale that does not fit through a door/staircase, it's a given you will have to re stretch a canvas. Here is a brief step by step DIY with basically what is involved stretching or re stretching a canvas.
When stretching a canvas, I start stretching with this triangular pattern. This method stretches the canvas and distributes the tension evenly, firmly and balanced. The same pattern is repeated gradually working around and out to the corners.
After stretching, if there is any slack in the canvas, I will spray the back lightly with a mist of water and let it dry. If this isn't effective and there are big pockets then I might have to re tighten individual secures.
The excess canvas is secured in the back so that the canvas sits flush on a wall and the corners are tucked and folded and secured like wrapping a gift. All is good to go and ready for hanging.