Renovating in landmarked building

The architect might charge a little more because it still has to go through landmarks but the construction will be exactly the same.

Assuming you aren't changing the exterior not sure why you'd need to go through LPC.

However, you'd still need to sign a https://www.hauseit.com/sample-alteration-agreement/

and of course, you'd need to get

https://www.hauseit.com/co-op-board-approval-nyc/

Super important to remember :-) Don't just renovate without asking!

The only thing that will need LPC approval is new windows if exterior otherwise isn't being affected. The building may already have a master plan in place for windows (meaning a universal style/color has already been approved by LPC for replacements).