There is an easier way. You do not need to use the grid at all.
Draw your proposed renovation at scale 1:1 in the modelspace (i.e. draw the side of your porch that you have labelled 25' actually 25' long). Then you can use one of the tabs next to the model tab labeled 'Layout 1' in your drawing to setup a window (called a viewport) that looks into your modelspace. The scale of the window can be changed (viw the properties pane) to present the plan at different scales (e.g. to provide a close up of joinery in the corner; this does not actually scale the model space 1:1 model though [which is good]).