How long ago was the work done? Maybe grandfathered in? But once you do work (to the areas in question) you have to bring things up to code.
If all you plan on doing is the furnace, sump and elect upgrades, it may not be necessary.
All that said, confirm what I say with your attorney.
But more importantly for your own protection, you do want to know that the work that was done without the permits was done correctly, your home inspector or a licensed builder (which I am) should be able to tell you based on what they can see if it would meet todays codes. Neither have x-ray eyes to see that the framing, electric and plumbing behind the walls was done correctly.
To bring things up to code can vary greatly based on the extent of the issues I've raised above. It could also cost nothing. PS I also see this work was done in the 80's so even if you were to replace a kitchen or the bath today, they would have to be brought up to today's codes. ie: GFI no in the 80's yes today. Bath ventilation 80's (window ok) today you need a fan.
This is not the place to get this question answered.
I would get 3 different contractors to estimate the work - after looking at the specific property in person.
I might even go so fas as to get electricians, plumbers, and structural contractors to give me estimates.
That way the tradespeople will be quoting without a general contractor's markup.
I would not rely on answers that you get with no first hand knowledge of the actual property.
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However, having said that, people can wildly overpay for bathrooms and kitchens. I recently rehabbed a house. It had two baths--one that needed to be gutted but nothing moved around. The other not only needed to be gutted, but things moved around, concrete jackhammered to put in a drain line, etc. The cost per bath was about $7,500.
As for the kitchen, we gutted that, ran new wiring, installed new lighting, moved wall switches around, put in all new cabinets, countertops, appliances, etc. We even had to move a wall. The cost was about $17,000. And we certainly could have done it cheaper.
Incidentally, the house I rehabbed sounds about like yours. It was built in 1950, and the previous owners had put in one of those baths around 1980. So--if you really proceed carefully, get multiple bids, check out your contractors, and so on--I would expect you could do what you're trying to do for about what it cost us.
Hope that helps.
When dealing with the building inspector it's important to convey that you are not the one who created the issue, but that you are trying to correct the problem in the proper manner.
Proceed with caution & make sure you're properly informed and covered,