Be mindful of the state of the property's general structural integrity as well as the roof, HVAC, plumbing, electrical, and door systems.Create a comprehensive scope of work that describes all the fixes and enhancements that are required.

Heating costs go up in winter, as does electricity due to the reduced natural light and more people indoors.

Second, look at converting to all electric. 

At some point they'll take over the business and do whatever they want with it (though 2 of the 3 already have the real estate "bug").

I'm also curious how much it costs to prepare the land and install plumbing, electricity, and concrete (driveway, walkways, patios).I'd love all the insights I can get from anyone that has experience doing this.

"wets" likes water, storm, sewer, and "dries/dry" like gas, electric, etc.

On top of that, you have other costs, such as permit, water, electricals, sewage system, etc... 

A 3rd floor addition will not only have structural modifications, but also require mechanical, electrical and plumbing permits.

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I also wouldnt worry about the electrical drain unless you are converting it to an MTR and paying electricity.