Any experience building on Flag Lots?

[Jeff T.]

We own a SFH on 1/3 acre of land that's zoned SF-3. We have a 4,000 sq ft rectangular patch of grass that I'd like to explore building an ADU on. The lot is "Flag Shaped" meaning that this rectangle of grass is connected to the main street by a thin driveway that's about 25 feet wide (ie. we have 25 feet of frontage with the main street). On either side of this thin driveway is land owned by the neighbors.

I was told recently that under current city zoning laws ADUs are only permitted when there's frontage of 40 feet or more. Has anyone else explored something like this on Flag lots before? Thanks in advance!