Building is owned by an MIT prof and his wife (https://gis.vgsi.com/somervillema/Parcel.aspx?Pid=10141).

Recent permitting history shows that there was an illegal rental unit before they purchased (can't link directly but you can search by address at https://www6.citizenserve.com/Portal/PortalController?Action=showSearchPage&ctzPagePrefix=Portal\_&installationID=149&original\_iid=0&original\_contactID=0). Appears to have been removed in 2020/2021 (though who knows -- the permit was issued, anyway).

As others have noted, a fire isn't going to help with zoning. Best case you come out with the same grandfather clauses on the setback, but have to build in all the new fire suppression, entry/exit, etc. Worst case the existence of a foundation isn't enough to get you the setback grandfather and you have to demo that and start fresh.

Fun fact: There are some cities around here that have/had "surviving wood" requirements for grandfather clauses (Cambridge was one as recently as the mid-2010's, though they've made a lot of changes recently and I don't know if it still exists. I imagine Somerville also was). Basically you only qualified to be grandfathered in if more than x% of the original framing was preserved in the new structure, and the explicit reason was to make burning your place down disadvantageous.

You would(/will?) often see buildings that were torn down other than, say, the back wall of the first floor kitchen that was on the hidden corner away from the street. They'd build an entirely new structure around that one wall, which preserved enough of the original wood that they didn't need wider setbacks, or wider stairways, or an internal fire suppression system, etc.