My approach to this would be to actually use two different classes for the 'wall'. The first class is the 'ghost wall', and the second one is 'real wall':

• click once, place construction marker, enter 'ghost wall mode' (something like just toggle keeping your mouse down in your app)
• you actually create a rectangle of ghost prefabs (could even just be a transparent version of the 'real' prefab) from the 'marker' up to the mouse cursor
• when you click again, you copy the 'path' of ghost wall to a path of real walls, so basically you can even possibly make both walls subclasses of 'wall'. then exit ghost mode.

Yes indeed i have 2 prefabs for walls , and marker is underwork, its not problem. But now main problem is how i draw that path from point A to point B and how to allways snap build marker on corner of tile?

I hope you guys can give answers !