While you are right that in general a home that backs onto Northern Blvd would be worth less than one that doesn't back on it, and it will generate less money in your pocket when you go to sell it in the future, there is no fixed dollar amount by which you could calculate such a difference.
On the other hand, the home that backs a main road will usually be purchased for less money as well! This could be your golden opportunity to live in a neighborhood that you want and be able to afford to live there!
To get a general idea of the difference in prices I would suggest that you check out homes that have sold in both locations, keeping in mind that these homes are not identical and were not sold at exactly the same time - so even had they been in the same locations there might have been a difference in selling price.
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Good luck and Regards,
Arlynn B. Palmer, ESQ, SRES, CBR, CMS, CNS
Associate Broker, REALTOR
Coldwell Banker Claire Sobel
Something you can't change about a house(in this case Northern Blvd) that can be perceived as negative affects the value the market will be willing to pay for this house compared to similar properties without this permanent situation. As Tracy said you will pay less for this house now, and you will sell it for less later. And homes like this typically take longer to sell.
To help you flush out the price differentiation, you should look at comps for the area...for homes that back the Blvd and for those on the same street that don't. There is a difference in price, but it should be calculated with current data for that street/area with homes of similar condition and size.
If the house offers all the features you want at your price point that is something to consider. The condition or overall size might outweigh the Blvd for you. If you don't want to back on the Blvd, at that price point you will have to sacrifice size, condition or both.
It comes down to what your hierarchy of needs at your price point and being realistic that the Blvd isn't going anywhere and will be there when you sell.
Hope this helps! And come back and let us know what you do next...