So, the city sees the trash bin and they catch him remodeling the entire building without permits.Now, that walls are opened in the building this fool finds out that the entire 25 units needs new water piping and new drain and sewer pipes underneath the entire building.

The development process starts with site selection appropriate for the number of units you would like to build, then you hire a civil engineer to do surveys, site planning, soils and environmental in order to begin the entitlement process, then you get an architect to design the product, then you get bids from multiple contractors.

The property is totally undeveloped, so water run-off, foundation work, soil testing all would need to be done. 

Prior to getting a soils report (geotech), you should have the an idea of where the house will be on the property, so the soil samples are taken from close to where the perimeter of the house is, otherwise you'll need to have to soils engineer back out later to take additional samples.

Look for a local soil studies or a geologist.

@Jimmy SamaiYou can also try bleaching the subfloor underneath and priming it as well.

Are there hardwoods underneath?

The ground underneath has now settled, so have them build up with 4" of gravel and compact it.

There's a high likelihood of contaminated soils

Hi Sylvia,If it were mine,  (and if I wanted to spend the money):I’d leave it all white and,Put black shutters on all front windowsGet rid of the cement circle in the front yard.Berm the driveway with soil to the grade of the cement (liability issue)Paint the utility box on the left side white to blend in.Paint the front door blackPaint the breather medallion at the peak black(Spray) white on the porch railing and uprights.Add some  low maintenance evergreen bushes to the left side that are just above porch floor height.Rewire the over garage light to 3 lights (1 to the right of the front door, one on either side of the garage) and use brass classic looking carriage lights.Convert the over garage light to a motion sensor with 2 flood lights for tenant safety and tenant retention.Put Large Brass house number to the right of the door at an upward angle.Good Luck with your decisions!