For a city with a Congressionally mandated height limit, you'd think this would be a softball question. After all, there's that big tall pointy thing you can see from all over, right?
It's not as simple an answer as you might expect. A lot depends on how you frame the question. Tallest building? Highest building? Tallest structure?
Clearly, my liberal arts major background was unable to deal with these technical differences, so I checked out the Council on Tall Buildings and Urban Habitat site, where among other useful details I learned that the formula for calculating the height of a residential building is H(residential) = 3.1s + 7.75 +1.55(s/30) where s is the number of stories. That's right, they came up with a formula to calculate the height of residential buildings (a separate one exist for offices, of course), where you know the number of stories but are unable to determine the height of the building. I'm hard pressed to envision a scenario where you couldn't find it online or, worse case, call up the property manager, but there you go. What type of dork could possibly find this useful?
But I digress, clearly these guys, who are the established authorities on determining what is the highest building, can help formulate the question. Not surprisingly there is a great deal of contention for the title of "World's Tallest Building" and the CTBUH actually has four categories to compete in: Height to Architectural Top, Highest Occupied Floor, Height to Top of Roof, and Height to Tip. No word yet if they plan to have a cage match to decide among the four categories, which I think shows a lack of innovative thinking at CTBUH and an elegant solution to a thorny problem.
So how does that apply to DC? Very little, but I was having fun with it. Let's take a look at the candidates for "DC's Tallest Building":
1. Hughes Memorial Tower: What the hell? What am I even talking about here? Well I'm sorry, but the tallest thing in Washington, DC is the Hughes Memorial Tower in the Brightwood (not Brentwood, as initially reported) neighborhood off Georgia Ave. in Northwest DC. Its 761 feet is hard to beat, but radio towers are dull and boring, so let's all do what the CTBUH does, and pretend they don't exist.
2. Washington Monument: Good strong showing and everyone's first choice. Its 555 feet 5 1/8 inches height is clearly taller than any building in DC. But, sadly, the CTBUH definition of building must "include at least 50% of it's height as usable floor area". So, let's leave the Monument with it's consolation prize of "DC's Tallest Structure" and move on. It can put it on its mantle, right between the "World's Tallest Structure : 1884-1889" and "World's Tallest Stone Structure".
3. Washington National Cathedral: This must be it, right? I mean, from the observation deck of the Washington Monument it dominates the skyline to the northwest. Its central tower, the Gloria in Excelsis Tower, reaches a healthy 676 feet above sea level, higher even than the monument. But no, those cheaters at the Episcopal Church built the Cathedral on 375 ft tall Mt. St. Alban's, meaning that although it is the highest point in DC, it's not the highest building.
4. National Shrine of the Immaculate Conception: Let's turn our attention to the northeast, to the tall tower we see that way. That would be the Basilica of the National Shrine of the Immaculate Conception, the Catholic Church's answer to the National Cathedral, although not the local Catholic Cathedral. At 329 feet, its Knights of Columbus Tower should be a shoo-in, and it probably is. Wikipedia, which knows everything, lists it as the Tallest Building. So why am I skeptical, besides of course from an undiagnosed psychiatric condition that makes me perpetually so? Because I'm not sure, and have been unable to find, anything that shows me that the Tower has 50% of it's height (164 1/2 feet) as habitable floors. So, assuming I'm right and Wikipedia is wrong, let's take a metaphorical leap of faith and see what's next.
5. Old Post Office: With office space well up to through the ninth floor, this clearly and incontestably counts as a building. It's 319 feet make it the third tallest structure, and if we can get a judges ruling on the Shrine, it just might squeak out a title win. Personally, I think it's the best view in Washington (best defined as what you see/hassle to get there), and it's certainly near most of the places you'll visit in DC. So, I'm pulling for you, OPO, even if you are 10 feet short.
As we wrap up a surprisingly complicated answer to what I thought was a simple question, we're still left with a bit of uncertainty. I suggest we ignore the titles, and enjoy each of these sites for what they are individually, fascinating cultural and architectural resources well worth your visit while in Washington. Except for that radio tower; it's just too boring.