[English] Sounds like your biggest problem is determining the area available for the garage taking all restrictions into consideration. Since your main interest is the garage I would go about it like this:
Draw thin walls representing the outline of your plot. Draw the walls outlining your house. Now draw walls or a room outlining the entire area available for your garage. No matter how odd the shape, the point is to see how much space is available considering the restrictions from your municipality. You can use the dimension tool to indicate distances from your plot border, house, and neighbors. Now you have an idea what area is available and you can design your garage inside that area. Don't forget that you will have to get in and out of the garage
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[English] Displaying the length of walls from the center of the ends has been discussed many time before. There is no good alternative. With inner walls and twisting corners it becomes impossible to determine what is inside and what is outside. But since you know the wall thickness it's easy to determine what either the outside or inside length is. You just have to add or subtract half the wall thickness for each end. Our fellow member Dorin created a mini-tip where he explains how to fine-tune drawing walls: https://www.sweethome3d.com/support/forum/viewthread_thread,9807#46003
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