Brooklyn's constellation of grids produces buildings at all sorts of flatiron angles, but occasionally one comes across a home or business askew for a reason that isn't readily available in the current landscape. Famous examples include the Hunterfly Road Houses in Weeksville, which sit at an odd angle to the Crown Heights grid because they were built along the Hunterfly Road, which no longer exists.
Another spot where the building lots are inexplicably off-kilter is the corner of Prospect and Underhill in Prospect Heights. You can see the strange angle clearly on Property Shark and Google Maps, slicing cleanly through the block bounded by Underhill, Prospect, St. Marks, and Washington, with the occasional spur onto an adjacent block. The angled zoning has produced a series of buildings at odds with the grid, seen above.
A legitimate explanation of this phenomeon would take more time than this amateur has, but I've done my best on the web (a call to the realtor of 115 Underhill turned up nothing) and come up with a trio of theories, none of which are mutually exclusive.
1. The Farm Lots theory:
Pre-grid maps of this area were hard to find, but I came across two that might explain the angle. The first was the 1846 map of Brooklyn farm lands above, which ran in an 1896 historical piece in the Brooklyn Eagle. It's not exactly to scale, but there is one lot that could potentially explain the angle. You have to zoom in to see it, but it juts nearly due east from the Flatbush Turnpike before the southern end of the property ends, at which point the eastern edge creeps north at a unique angle (for the map). Another map I found of farm lots superimposed over the modern grid further west in Crown Heights showed the farm lots at similar angles to the grid.
2. The topo theory:
This Brooklyn subway survey map from 1888 has topographical lines, including one that slices right through the area in question at a similar angle to the existing one. This doesn't mean that there was anything special there, of course, but it does demonstrate that the hill slopes at that angle, which might have contributed to farm lot, and later residential, zoning.
3. The older-grid theory:
I couldn't discover when various streets were laid, but I've always noticed that Washington Avenue is an odd carryover of the Bed-Stuy grid into Prospect Heights. This 1855 map shows the streets as currently gridded, but perhaps there were briefly plans to grid the area differently. This might explain Washington's angle as well as the line of these lots, which make almost a right angle with Washington, as the streets in Bed-Stuy and Clinton Hill do. The Flatbush Turnpike also used to cut through the neighborhood closer to Washington (you can see the angle of the turnpike in the 1846 map above), before slowly migrating west to become Flatbush Avenue.
These are completely inconclusive research results, of course, and they don't offer much of an answer. Thanks to the Brooklyn Geneology folks for their great maps page.
Update (10 minutes later): This is frustrating, but here's the answer, as complied and blogged by Josh Jackson of LOST magazine. A bit more googling finally turned up his piece, but I can't bring myself to take this pale shade of it down. Apologies for anything that seems like plagarism (though I'd be a poor plagarist if this was the best I could do).