If you live in a loft or other building with a concrete slab ceiling or your renovation includes making the joints between new drywall walls and old plaster walls appear straight. Those slabs and exiting walls are unlikely to be flat and require “flanking” or “floating” to straighted out the lines, again – if that is what is important to you.
Some architects have stopped writing “level 5” finish for just the reasons above, replacing that specification with 1/8” skim coat or they write that the new and old walls will blend “seamlessly”. These specifications make more sense if the purpose is more than just to eliminate the texture difference between the paper face on the drywall and the compound tape joints.
Even with the above specifications, some conditions should be specifically addressed and communicated to the bidders such as:
If the previously mentioned joints between a concrete slab ceiling and the walls are to be straightened.
Fiberglass meshing over shattered plaster walls or plaster walls with electrical channeling
Straightening out corners or wavy soffits in prewar buildings (they can be out of square against new cabinetry)
Anywhere a high gloss or lacquer type finish is to be used
Other surfaces that can be skim coated such as moldings or doors that have been rolled out and where a brushed or sprayed finish is desired.
On the other end of the scale, removing the texture in walls with 50 years of paint or just a bad paint job, sometimes referred to as orange peel, will not require 1/8” of compound so it is important for your architect, designer or contractor to fully understand what you want to achieve. If removing texture is all that is expected, words such as “skim coating will be done to remove texture only, walls will be smooth but not flat” will work.
For budgetary purposes in New York City, figure on skim coating adding 100% to the cost of a paint job but that is just an average, sometimes its more, sometimes less. What you get for the money is new or often better than new walls and ceilings. But, if existing conditions or specifications are not addressed before pricing, you may wind up with a bargain price only to be faced with additional costs or a job you are just not happy with in the end.