The Big Bang Apartment extension is a modification of the official set 21302 and designed to be compatible with MOC Friends Apartment from the same designer LEGO Artisan.  It consists of 1028 pieces, of which 594 are extra parts that need to be collected aside from set 21302.
The layout is not accurate to the TV show. Rather, it’s been modified to fit the layout of the MOC Friends Apartment for a seamless incorporation as an intermediate floor between Central Perk and Joey’s apartment.
Before purchasing this MOC, it is advisable to look into the Friends Apartment MOC first. Once purchased, the builder will have access to the following:
PDF instructions
XML file for the complete parts list
XML file for additional parts list
PDF screenshot of additional parts list
Powerpoint guide to upload additional parts list to bricklink.com

Note: the instructions for the Friends Apartment is sold separately.

Changelog:
18-Jul-21: Updated additional parts list to include 3pcs white 1x4x6 door frames

30-Jul-21: Updated step 200 (page 127)