Mouse Drag event with hand extend width until drag stops

Hello, I am working on a project where a building is rendered after few inputs but once it is loaded, it’s width can be extended using a box attached to the end of one side wall, but I have tried many ways but the difference between start point and end point is varying every time some time it is negative / positive. Whereas, I also want to go draw service while the wall is being dragged using mouse cursor. Can anyone suggest me the logic and way to solve this. Thank you.

Can you share a Playground with what you have tried to do? It will be easier to help this way.