Those were inclusive of furniture pieces for the 3 bed 2 bath plus office, stock of sheets, towels, beddings, and accessories in each room, as well as supplying kitchen materials and bathroom essentials.

They ended up valuing it as a SFH with the ADU as an accessory and not added to the square footage.

According to the zoning chart, non-owner occupied STRs in commercial zones are covered as - PC Permitted subject to certain conditions (Ch. 17.16)Owner occupied STRs in commercial zones are covered as - A Permitted as accessory to principal use (Ch. 17.16)So if you are doing non-owner occupied, you need to find out what certain conditions need to be met in order for you to do it.Personally I would avoid Nashville as it isn't terribly STR friendly and there is not sign it will get better.

$200-250k doesn't buy much multifamily in most markets so I would encourage you to consider purchasing a single family residence with an ADU (accessory dwelling unit or mother-in-law apartment) as well as small multifamily options. 

Background info: A+ area (wicker park), 3 large 3 bed legal units, 1 non conforming, 5 minute walk to El, quiet desirable tree lined street, huge ceilings, tons of light, original millworkCurrent Rents: $7220 (3x $2000 + $950 accessory unit + 250 garage)Price: Under contract for 1M, expecting about 30k in buyer concessions (nothing major, but seller indicated flexibility to address anything)Pros: 40 year owner, new roof, new windows, new garage, recent boiler/water heater - stout mechanicals, easy to duplex down into basement to make 4bed / 2bath 2250 square foot unitCons: window AC, radiator heating (3 separate meters, but one tied to boiler - owner covers heat), single coin op laundry, porch in usable but older shape, older kitchensComps: $1.15-1.25M for similar but upgraded rental (not condo standard) units.Option A: Do nothing:If I were to do nothing at all and keep rents (below market atm - at 20% down (interest rate is unfortunately very high, but such is the time - 5%) with 4% loaded for fees, $200/month for radiator gas in entire building, it's cash flow $0, CoC $0.Now of course you might be screaming this is a terrible deal - I think there's a lot of value add here for a unique building in a prime area. 

This means you can rent out just a room while you're there, you can rent the whole place while you're gone, or you can rent a basement apartment or accessory dwelling unit such as a carriage house.

Has anyone successfully built an ADU in Washington DC?

The wording below from the ADU code you sent(thanks) got me wondering;"Either the principal dwelling or accessory apartment unit must be owner-occupied for the duration of the accessory apartment use"Thanks, Loid

I'm also considering SFH or townhomes in VA, MD, hopefully close to metro lines or public transportation, with an accessory dwelling unit.

Of course, your costs may be more or less, depending on what you want for your accessory dwelling