What even is this?

The beginning of the description should about cover it, but some further explanation never hurts. This is not something that changes the game world in the traditional manner as a "proper" stand-alone mod would do. This is a Workshop Framework layout you have to apply yourself after you have unlocked Hangman's Alley as a settlement. You absolutely should make a save you can revert to before you do that. For many, this should be something they download, explore for a bit, then uninstall and proceed playing the game from a point where they never applied it in the first place. Hence the making a save part.
You can still use it as a permanent addition to one of your playthroughs without any problems, as long as you don't mind the performance hit of the disabled precombines in the area. Furnish the interior to your liking, house settlers in it, modify my design as you see fit, etc. But be warned: this upload doesn't readily include any way to revert the buildings back to their former state once the layout has been applied. There is a way to do that, it is not particularly hard to do, and we'll touch on that in the "How?" part of this article. Me not including it in this upload simply stems from sacrificing practicality in favour of a certain awe factor and trying to push the limits of what is reasonable. This is the "tech demo" part of this layout. It shouldn't keep any of you from including it in your future work, should you choose to do something similar. Again, I'll tell you all about it down below.

Why tho?

Adding interiors to buildings is kind of a big thing. Every new interior expands the game world, little by little. This file aims to demonstrate how anyone can do just that now, without using any external tool (not counting the online layout creator provided by kinggath). You have to disable the precombines in the area in order for this work, and the affected objects have to be within settlement borders, but those really are the only two limitations. Build High - Expanded Settlements has got you covered as far as the precombines go and also helps alleviate the settlement area issue. In addition, mod added settlements also work, as long as both the creator and the user has whatever mod that adds said settlement installed, but in that case you need to use something else for the precombines, if the mod in question doesn't already cover it. The .ini tweak is your last resort here, but it does work. With the right setup, Scrap Everything lets you remove things from the game world without the need to edit the cells themselves. Scrap the bloody ground if you want to. Hell, scrap parts of the floor in Home Plate to make room for your basement entrance! No, really, I tried it, it works. Finally, you have no need to create your own vanilla styled building blocks to rebuild your targeted building with, Snappy HouseKit already does that. So get to work!

How?

Pick your building.
Assuming you have all the required mods installed properly, there should be all kinds of buildings within your reach that are now made of smaller parts you can scrap, move around, place, etc. These parts should have almost exact counterparts in the menus you got when you installed Snappy HouseKit. I have yet to find examples where this is not the case. Once you've confirmed you have access to all the necessary building pieces, make a smart choice. The main reason this upload of mine doesn't include a way to easily revert to the vanilla state is the size of the layout itself. More specifically the number of objects. It would take half of forever to remove all the stuff I placed. So pick something that doesn't require too many parts to replace. I'd also suggest foregoing any furnishings or decorative stuff for now. What you want is simply a building that feels real, with real walls. So limit the layout to that. But don't go scrapping yet!

Create a scrap profile.
Since you have precombines disabled, you need to treat your settlement as if it was a custom (i.e. mod added) one, as far as Workshop Framework is concerned.
I'd suggest creating separate save files during the stages of this whole process, in case you need to return to any step in the future.
You need to make 2 exports of your chosen settlement with the scrap profile option in your settings turned ON. They need to include every little object in your settlement, so it may take a while.
The first one needs to be of an untouched, unmodified settlement, in the state that you found it in. Let's call this your ScrapBefore file.
The second one is from a settlement where only the exact things you want scrapped are scrapped. Including your building of choice. This'll be your ScrapAfter file. This enables WSFW to remove the exact things you want gone in your layout. There are a number of ways to get to this state, I found it the easiest to begin building only the new exterior of the building to see what exactly I need removed. There might be vanilla stuff that you don't want to bother replicating during the building process, because no one will ever notice they are the original parts. Saves on time and resources.

Build and make the final export.
As it happens, these newly scrappable buildings do include some snap points you can use, believe it or not. A nice and easy way to go about this is finding such a point and snapping the corresponding wall/roof/whatever piece from the Snappy kit right into the same position where the vanilla piece is. When you are sure you got it into the same position, you can use your new piece as a starting point and start replacing the neighboring parts from there. Scrapping and building, piece by piece, until you've got the whole building. Then you make your interior walls, floors, whatever. Once you're done, you may do your export with the scrap profile option turned OFF this time, since you already taught WSFW what to scrap and what to keep with the first 2 files. This should be a much quicker process, it doesn't worry about all that information about everyone little object, only what you have built.

Congratulations! You can now feed your ScrapBefore and ScrapAfter files alongside with your last export to the online layout creator tool. Any updates or modifications from now on only requires exporting once. As long as your ScrapBefore and ScrapAfter files are accurate enough, you may reuse them over and over for such purposes.

Be smarter than I was.
It was brought to my attention that this layout brings back some vanilla items, should they be already scrapped. While this really is a feature of WSFW, and definitely not a bug, bringing along these specific items was not my intention. For this reason, it is best to be thorough with your exports. This is the whole purpose of the scrap profile. I'd also suggest making full use of Workshop Plus - have the stuff you want to build on a separate layer, and include only that in your final export.
Alternatively, you can use the "create differential" option to ensure your file only builds what you want it to, excluding miscellaneous vanilla objects the exporter might have picked up, mistaking them for something you have built. You do this by subtracting your ScrapAfter from your final export. The resulting file should only contain objects you placed manually. Using this file as the main layout file (what would be your last export under normal circumstances) is probably the safest option. These steps are necessary thanks to the complications that can come with disabled precombines. However, taking this last step also ensures compatibility with other layouts that also remove stuff via Scrap Everything. In other words, the resulting layout only removes what you want it to remove, and only builds what you want it to build, with information about what to scrap and what to place stored in neat little categories. If you're familiar with Set theory, it's basically that.

Restoring the vanilla building
I haven't tested around much with this functionality, but WSFW does have the ability to restore vanilla items. There might be a better way to do this, but theoretically, my solution would go:
You use the "create differential" option of the online tool to subtract ScrapAfter from ScrapBefore. This leaves you with a list of objects that only exist in ScrapBefore, but not in ScrapAfter. These should be the exact objects you have scrapped. Then you go on and generate a layout from this resulting file, telling WSFW that these are objects to build. Give this a name indicating what it does, like RestorationLayout.esl, or something along those lines. At this point, all you have to do is disable your original layout in-game, then choose and apply your RestorationLayout. Do tell me how this part goes. I have not personally tested it, but it should work.