I stumbled upon this by accident. I was taking photo on some side streets of places with flags in front of them thinking they might be Embassies.  This was tucked in... a very small looking row house with a small plaque.  Very unassuming looking. In 1913, when Woodrow Wilson appointed FDR assistantsecretary of the navy, ER and FDR moved to Washington, DC where they rented Auntie Bye's home at 1733 N Street, NW. In autumn 1917, they rented a larger home at 2131 R Street, NW."

"In 1912 F. D. R. supported Woodrow Wilson for the presidency and Wilson won. Wilson appointed F. D. R. Assistant Secretary of the Navy. The Roosevelts moved to 1733 N. Street, Washington D.C.

"Beneath its canopy, then-Assistant Secretary of the Navy Franklin D. Roosevelt's affair with Lucy Mercer blossomed in 1916-18. Eleanor Roosevelt's discovery of FDR's dalliance from love letters in their home at 2131 R St. NW was the devastating blow that launched her transformation from meek helpmate to icon of female independence."

"Franklin and Eleanor Roosevelt lived at 2131 R Street, while Roosevelt was Assistant Secretary of the Navy, a significant launching pad for his campaign for Vice President in 1920 (see box). Across the street from the Roosevelts, at 2132 R Street, was the home of Attorney General A. Mitchell Palmer." Mr. Palmer's residence was bombed and body parts flew very far (bomber's body) with one limb landing on the doorstep of the Roosevelt's home.

I know there are tours in DC which may include this house, but I am not sure it's a home you can "go inside". I think it's privately owned now or part of an Embassy.  However, it doesn't hurt to inquire.