.); no more than 60-70 m2 (that's about 650-750 f2) to, among other things, lower renovation costs; 1-2 bedrooms, street facing and with a lift (if said apartment is above the second floor); with good bones, i.e., needs only partial renovation.

I am looking for help with estimating rehab costs and to help during the rehab period... 

They said we low balled them, I told them they didn't and guess what, they received another offer near ours and at the end, the property didn't sell.Construction costs are high.

I haven't seen anything under 200-250k all in cost 

Costs are up, but so are asset values and potential rent increases.

If you refi to a renovation loan by including the ADU costs + paying off the existing mortgage, would your total monthly payment be financially comfortable and agreeable to you? 

Additionally, consider cards with no annual fees to keep costs down.

OK, I would urge you to be very, very careful here...Certain operating expenses are paid monthly from lender escrow, so it's easy to assume that any costs not paid this way are somehow "optional."

North NJ is very attractive for many reasons, one of them is its convenient transportation to the NYC. 

I wouldn't consider this an investment, but maybe a way to offset your costs.