Good question! You can check with Montgomery County Park and Planning Department to get the answer.
In the meantime, have you already selected a new house floor plan? Sometimes, it's best to do that first. There's nothing worse than buying a home site only to find out later that your dream house won't fit on it.
Also, besides the lot and custom home prices, make sure to budget for interest-carry on your construction-permanent loan and the various site costs (clearing trees, laying driveway, connecting to utilities, etc.) which can range $20,000 - $120,000 depending on the home site you select.
I'm a broker and specialize in land acquisition and new custom homes in Bethesda and Potomac. If you'd like a free consultation, please call me at 301-440-4805.
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A good architect who works in the area will also be very familiar with the zoning restrictions and what might be possible in lot coverage. They are worth the money invested before you make any purchasing decisions.
This document (found on the mctx.org site) lists applicable setback requirements, lot coverage ratios, etc.
There are plenty of singel family homes built in Bethesda on less than .15 acres so the subdivision in theory is doable. However, the county can always require special conditions with any project above and beyond the basic requirements. (i.e. storm water runoff control, etc.)