Can't add chairs - Don't fit slot size

@fuanonemus  That is definitely strange that the base game lots are impacted by this issue. 

Here is what I recommend.
Go to Documents\Electronic Arts\The Sims 4

Rename The Sims 4 to The Sims 4.old to preserve your saves, tray, screenshots and mods folders/files.
Run Repair from Origin as described above.

Start the game without adding anything back as a test.
The game will automatically create fresh versions of all the folders/files.
If the game works correctly you will need to test your mods /custom content (CC) to see which they are causing the problem.
When adding/testing/deleting mods and/or cc it is important to delete the localthumbcache.package and any other cache files.
Using the 50/50 method is suggested to speed up the process.

Instructions are in the link below:
Troubleshooting tips
https://answers.ea.com/t5/Technical-Issues-PC/READ-FIRST-Troubleshooting-your-Game/m-p/3621199

EQ Avid Sims Player
#EQCreations
I do not work for EA
All packs installed and patches are current.
PC / Win 10
Never used CC or mods.