This is the grand table that started me on updating all my equipment tables to begin with. It is by no means complete ... there are so many aspects of carpentry that could be put on this table, however, that I would prefer to wait until a player asked for that item's cost before I bother to include it. For that reason the reader won't find a flight of stairs, cabinets, shelves, balistrades, a balcony and so on. The stairs alone of that list are included in the costs for two and three story houses ... but if you wanted a separate staircase, I'd have to work out the size and give you a price. The fact that the stairway isn't included here doesn't mean it can't be bought. Everything can be bought. When I post this table again a year or more from now (when again I upgrade the algorithm and add additional features), there may be more here. Once something is worked out, I leave it on the table.
There are several features about housebuilding that need to be discussed. First of all, the difference between "designed" and "existing." This should be obvious ... but I'm including a different price for each because the players should not always have to build something from scratch. If they want a residence in town, the price is included. The older the house they buy, the less it costs up front ... but maintenance must be spent to 'bring the house up in value.' This maintenance could be physical labor and time spent by the character, IF the character has those skills necessary. This would save costs. Obviously, not all existing buildings will be in towns ... but most wooden structures would be. Stone structures are available at the mason's, which will get posted later.
A structure can be slowly brought up in quality by spending twice, three times or more of the usual monthly maintenance cost (assuming 1% of the total cost being spent per month, straight out of the DMG). It really depends on what the player's want to spend, and whether or not they want to try to do work themselves, or hire others. Obviously, the latter is more expensive. Oh, and incidentally, it only takes one carpenter to oversee the work of four laborers ... so as long as there was one carpenter in the party, the work could still be done by five persons altogether.
Please note also that in addition to giving the price and volume of various items like paint, pitch and sealing wax, the actual application of the item is listed also. It is totally stupid to list a price for paint, for such and such an amount, if the actual amount of work the paint will do isn't included! These paints are waterbased, by the way, this being the 16th century. Mostly it was water mixed with lime and pigments. Oil-based paint was rarely used for housepainting (note to self, create prices for oil-based paints at the printer's shop).
I grant the gentle reader that the measurement of an outside wall is an inexact method for determining the price of the floor and roof also, but please trust me - you don't want to spend a lot of time being particular here. This is one thing that can be glossed over with a degree of simplicity ... I have tried the other way, and unless you have a party that wants to do the leg work for you in determining the exact amount of wood needed, save yourself a step and use the general estimate. The price won't change all that much with exactitude, and the time saved for the campaign (and the ease of players to make simple calculations for their own abodes from the table) are worth the approximation.
But I did include the lumber option if a player wants to be exact and build the house from scratch. Why not? The reader might remember that nails are available at the blacksmith's, and that a measure for how many nails are needed for such and such an amount of board feet is also included.
Oh, nearly forgot. The house may be built by the carpenter, but the price of a brick fireplace and a concrete foundation are included, so a designed house would involve a visit by a mason and bricklayer. The total tonnage of the house is included (weight of materials) so that the amount of moving and shifting can be calculated by the player and DM.
I'm afraid I haven't sat down and worked out the approximate building time for houses. I hope to add this, but I admit I don't know what the answer would be. If someone wants to offer me a simple calculation that would include the transport of the materials (with distance from source) and the time required for assembly and rough finishing, I would certainly consider it. Problem is, most times I get unusual specifics that are not universal for all wooden construction, and its the universal measure that's needed. It tells me nothing to know that a particular house takes this much time. How does that help in defining how long it takes to put up a two story barn that is half the size, or twice the size, as the one listed here?
Oh, and the question of why someone might design a three-story house but not a two-story house, noting that the latter is not available on the table - perhaps it is a desire to only take contracts that are worth taking. "If you're not going to give me work for three floors, goodbye - there's only so much season I have to work in."
One cannot be certain of the motivations of others.