Great replies already! I just wanted to add a few supporting points:
1st: At your price range, your prospective buyers will likely be moving 'up'. If they search for a bigger place with 2 baths, your home won't even come up on their search and they will not go to view your great features. I definitely recommend adding a full bath. You'll open yourself to an entire pool of buyers.
2nd: Baths and kitchens still sell. They aren't the only factors but they are memorable- from 'wow' to 'eh' to 'on to the next one'.
3rd: Buyers really overestimate the costs of remodeling and repairs. They may not have extra funds after the purchase or more so (now) just want to keep house shopping where it isn't an issue.

You will increase your buyer pool and increase your price by adding a FULL bath.

p.s. I know a great plumber (he's my personal go-to guy).

I'm also a certified Stager. In the past, seller's have increased their net return by making minor changes ahead of time to increase their price by 2-10x what they spent. I'd be happy to offer a preliminary consultation- angiedelboy@mris.com

Thanks so much Don, I will definitely look into getting some estimates!

Two responses: In general, adding a second bathroom is a wise decision, and frequently comes close to recovering its cost, if not exceeding it. (There are very few improvements that actually return 100% or more of their investment. Adding a second bath can be one.) Of course, it depends on how much money you spend on it.

Second: It depends on what homebuyers want and expect. I searched the MRIS, and most of what's available there, and what's sold, have had 2 bathrooms. I searched for 3 bed and 1+ (1 or more) baths. Currently, there are 3 active listings for 3/2s; average list price is $489,933. There are 3 under contract, average list price $379,333. There are 2 that sold within the past 180 days; average is $448,000. There are 3 listings that were withdrawn; average price was $466,800.

As for 3/1s, there are/were almost none, even going back a year. There's 1 on the market now at $350,000; it's been on the market (on and off) for 478 days. It's a fixer-upper, right on Annandale Road.

Without having visited some of them, and without knowing where yours is, it's impossible to tell for sure whether adding a second bath would return its value. You also have to keep in mind, though, that even if it doesn't return 100% of its value, if it helps your home sell, say, 60 days sooner than it might with only 1 bathroom, that has a value, too.

Consult with several Realtors. Get their opinions. But, just from what I know of the area and looking at the comps, and having seen the outsides of some of the houses, I'd say you definitely should pursue your investigation. It may well be worth it.

Hope that helps.

Thanks! I do have a rambler. I am thinking of adding a half-bath on the main level across from the coat closet, or taking some room away from the laundry room to add a master bath (which would be a lot more work/money for sure!). So it would be within the existing floorplan. No estimates, if it wouldn't recoup at least its value, then I definitely would not go even that far. I am just wondering if the idea is worth taking any further.

It sounds like you probably own a rambler. Would the bathroom be in the basement or on the main level? Would it give a private bath to the master bedroom? Do you have space for it or would you have to put an addition on your house? Do you have an estimate on the cost of the project?

The answer would depend on those things.