Wouldn't that screw up the whole "loading docks delivering parts directly to the assembly line workstations" layout? Unless they have loading docks on both sides of the building?
There are loading docks on both sides of the building - but I think the angled/sawtooth loading docks on the other side are for the BIW lines.
But my guess is that there is so much overlap in final assembly of the Model Y and Model 3, that I'd expect the two lines to run in parallel and next to each other, and workstations to be 'mirrored' in a fashion - and maybe even shared and crewed by the same people, and the parts prepared and conveyed in unison. Given that throughput is about 1 car every 3-4 minutes, it's not a problem for supplies to cross the assembly lines - as long as the second line is still close to the loading dock and the unpacking area.
The Model Y shares 76% of the parts with the Model 3 - and I guess a large part of that percentage is the chassis/panels, which are assembled by robots. At the general assembly level, which is about ~70% of the factory footprint of total car assembly, I'd expect the part sharing factor between the Model 3 and Model Y final assembly lines to be as high as 90%, maybe more.
If that is true then it would be natural to line up the two assembly lines and build them in parallel, and only bifurcate the workflow where there's real differences.
The Model Y might still require a new building: a BIW body line next to the stamping shop. The new pile-drivers are close to such a location. This new Model Y BIW building would be relatively small, compared to the length of the Model Y final assembly line.
But even this layout isn't ideal: it would mean that the Model Y bodies would have to travel the whole length of the building to get into the paint shop. So either they build a second paint shop for the Model Y, or the current Model 3 BIW line has enough capacity to make the Model Y as well:
Note the very purposeful topology of the assembly lines: stamp shop feeds into the body shop on one side of the building (which has the saw-tooth pattern loading docks), which feeds into the paint shop, which feeds into the start of the very long general assembly line, which spews out finished cars into the parking near the stamp shop. The car, as it is assembled, takes an almost perfect circle within the factory and always stays close to loading bays.
Note how the phase 3/4 building is a
mirror image of the phase 1/2 building: the highest throughput loading docks are at the outside boundary of the factory, and the dense, heavy shipments of raw metal to the BIW ships are loaded via the saw-tooth loading bays in the middle of the factory.
Just ... slapping a Model Y building next to the stamp shop breaks all this current and future symmetry, disturbs the smooth flow of cars within the factory. This makes no sense to me. I think both the current BIW and the general assembly line has enough free factory space to host Model Y assembly.
Could be wrong though. Maybe
@Krugerrand has some ideas about how they might approach this?