It can save space for living square footage and actually add value.

We are considering if there is adequate ROI to finish the basement and increase the square footage of the house.

Also, total square footage is important.The only thing that made me nervous about this setup is you may want to confirm that the unit in the back was legally constructed.  

After this addition/rehab, depending on the sq footage, in the current market the house would be valued somewhere in the range of 500k conservatively...probably more honestly..In other words we'd still have equity to weather a market downturn and make a nice tax free profit when it comes to selling the home.If this is done, we can stay long term 15-20 years if we wish...and have our home exactly as we want it.  

I created an Excel spreadsheet and began analyzing each zip code of my city for neighborhood, age of house, condition, square footage, rooms, baths, exterior building materials, slab vs. crawl space, garage, basement, etc. and then compared it to asking price, selling price, and time on the market.

On my electrical contractors forum we have some video of a couple thieves plus footage of the arrest.

I usely focus on the six months and the dollar per square footage before I even take in to consideration the property.

My initial offer is 50-60% of fair market value minus carpet and paint (I estimate based on sq. footage).

Bedrooms, bathrooms and square footage below ground do not get counted for comparison purposes.

The lot size is approximately the same but one side the houses have 3 bedrooms and a lot of yard, the other builder built 4 bedroom houses of a bigger square footage on the same lot footprint but the bigger house wth better upgrades leaves less usable yard.