You'll get a sense of it's "$5/sq ft for LVP flooring" or each window costs this much, exterior paint job is $6000, permits, etc Add up every little thing including light switches and electric outlets. 

Then you will have to think about the added cost to possibly modify the exterior of the house to accept the new windows.

Should be an easy fix to add a side gate, sliding glass door, french door, or regular exterior door and steps/patio. 

In 2018, I incurred significant improvement cost on a second property I own free and clear: replace flooring, paint entire interior, replace gutter, window frame, exterior door, new roof (done Dec 2017), electrical, plumbing components, and appliance upgrade.

I agree there is some cost savings and efficiencies with MFs ... one roof and exterior walls.

The architect can provide specifications for various interior and exterior finishes and materials.

Exterior spraying I do hire.  

The tub/shower sits along the exterior brick wall.

I was about to start the process to get a retroactive permit but realized the additional cost of a new exterior stairwell (metal) and architecture fees is almost 30kIm starting to weigh the options of either full tear down with a new 4 unit construction or separate building in the front of property (current is set back thus the extra costs to permit)  or adding a 2 story 2 unit addition connecting to the old propertysorry this post has become 2 ideas i need advice onif it deconvert can i use it as an inlaw suite with 2nd floor unit or a duplex up. 

It's time to do some cap ex repairs and update the exterior siding possibly roof and flooring.