TL:DR Adding some new information to some brilliant previous threads showing that the construction site (otherwise known as the "Kill room") and the base of the Square Form designers/fabricators was sited at the Pegasus building. Below is my attempt to explain how I believe this to be true.
This great thread https://voat.co/v/pizzagate/1499872 was posted by @spoor a while ago giving lots of brilliant information and analysis on the Alefantis owned "Pegasus" building at 3518 Rear 11th Street NW. Spoor concluded that this pic https://i.sli.mg/B0nf9D.jpg of John Kreher and Matt Proper looking at a hole in the ground, and this pic https://i.sli.mg/F2wqnz.jpg of two workmen breaking the ground up were not taken at Comet Ping Pong, were not taken at the Pajama Factory, but were in fact at the Pegasus building.
Yesterday I found this website http://www.margotwork.com/about/ of Joseph Margot, who worked for Square Form https://www.washingtonpost.com/news/going-out-guide/wp/2014/07/10/from-the-ground-up-running-on-fumes/?utm_term=.add70a3c5617 http://archive.is/q4zIV
If you look at the picture in the workshop from Margot's site, noting the area I have highlighted with a Red circle http://imgur.com/a/ec9xe
You will see that it matches this one from the construction pictures, also highlighted with a Red circle http://imgur.com/a/YH2Vi
and in-turn the brick pattern of the wider construction picture http://imgur.com/a/OmA2w
Now, how do we know this is at the Pegasus building and not Comet or the PJ Factory? spoor covered it in his thread, but I'll put a little here once more for ease of analysis:
This image shows all the relevant parts very well https://i.sli.mg/KNWZ7C.jpg
Now, the clincher is that for this construction site to be the work room and be at the Pegasus building, then there needs to be some proof of Square Form being there. Thankfully, we have that.
In these images https://sli.mg/a/s9hoRx from Joe Wills (Head of Square Form) we can see their former office space that was featured in an instagram picture (I can't find that picture at present).
Now compare those images with the images of the Pegasus building from the outside noting the locations of windows/doors etc as well as the new windows etc.
Older exterior view https://firstname.lastname@example.org,-77.0291146,3a,75y,56.01h,87.18t/data=!3m6!1e1!3m4!1szbiyw5pRNHLFIwbtPqEZbQ!2e0!7i13312!8i6656
Newer exterior view http://imgur.com/a/WwxwN (probably best seen in this video from @slackerjacker http://www.veoh.com/watch/v114980589gQE2RZdA )
Windows match in relation to doorways, window types etc.
And just in case you're still not convinced;
Here's John Krehers workmate Amos on a lunch break outside the Pegasus building garage door https://www.flickr.com/photos/146343979@N02/30802654726/
Pic of Square Form employee from JimmyComet's instagram on the other side of the garage door http://i.imgur.com/6MVGsAZ.jpg
Further pic of the inside work room of Square Form https://www.flickr.com/photos/146343979@N02/30751369911/ note the CPP hoody
Further pic in the work room http://i.imgur.com/lUC70An.jpg
And finally, a pic of an event being held in the Pegasus building https://www.flickr.com/photos/146343979@N02/30204135713/ (as you look at it you are looking from the front of the building towards the back) compare the rolling vault door in the bottom left corner and the rolling Mezzanine that the people are sitting on with the earlier pics from Joe Wills.
I'm editing in further evidence as to why I believe the suggestion that the former McCullough Construction building - 5039 unit 7 Connecticut Avenue - is the site of the construction is false. Politics & Prose hold regular book club meetings there http://archive.is/4WZuU http://www.politics-prose.com/bg/lez-read/lez-read http://archive.is/r8JSk
Confirmation of a basement at the Pegasus location https://i.sli.mg/3cl2Sy.jpg thanks to @redditsuckz