Some proposals simply say to paint the walls and ceiling and never specify the number of coats to be applied. If the colors are similar enough, it's possible to get away with one coat of paint and not discount your pricing. No matter how hard you try, tiny, pin-sized air holes will pop exposing the original walls. This may not bother you if you can't notice it, but principally speaking you should have paid your painter less for the work.
The final cost will heavily depend on the size and nature of the painting project. For estimating the quantity of paint required, you will need to know the room wall sizes (length, height, and width), the number of windows, size of windows, the number of doors and size of doors. A standard living room (12’x8’) will require 2-3 gallons of paint using the industry average of 350 sq.ft. per gallon. Depending on the brand of the product and finish type, the cost can range from $20 to $100 for two gallons. If you plan to have a different color on the ceiling, trim or an accent wall, that will have to be calculated separately and would again add to the final cost.
If less than half the old paint is left, however, it may be worth stripping it all off. Guertin gets rid of stubborn remnants using shrouded grinders (like the PaintShaver), infrared paint strippers (such as the Speedheater), or chemical strippers (like Multi-Strip), then smooths the wood with a course or two of sanding. When siding (or bank accounts) can't take the shock of a total strip job, Rich O'Neil, of Masterwork Painting in Bedford, Massachusetts, has successfully hidden rough, well-adhered paint under Peel Bond, a thick primer.