Checked Google Street View. It's clearly a 2 story building. http://sli.mg/X5Af86

Notice: There are windows on the upper level of the Construction Co. adjacent to Comet Ping Pong.

So, we know the building is 2 stories tall.

Let's look at an interior shot of Comet Ping Pong: http://sli.mg/HM21U6 It appears that the front of the restaurant, at least, takes up the whole vertical height - so where's the second floor?

I came across this tripadvisor review page for CPP. https://www.tripadvisor.com/ShowUserReviews-g28970-d940774-r116112239-Comet_Ping_Pong-Washington_DC_District_of_Columbia.html

The reviewer states "Try for a table either upstairs or outside on the small patio; even being confronted with Connecticut Avenue traffic noise will be an improvement over the din that can sometimes inundate the main dining area." http://sli.mg/1AYIdK

Additionally, the "basement" that was in Mr. Alefantis's instagram clearly shows an exterior door that opens to broad daylight, indicating that it's a walk-out/ground level basement. If you look closely, you can see that this part of the building, the back "curtain room" (notice the few stairs on the right) is slightly sunken. http://sli.mg/rvHxOy

What I haven't seen thus far: The kitchen/prep area, which is located adjacent to the pizza oven. (big round thing in the middle of the restaurant) The kitchen area should include a walk-in freezer and liquor storage. The "upstairs" dining area referenced in the tripadvisor review.