There are numerous books available to help with estimating construction projects and none of them are totally accurate. Estimating is an art and something that takes a long time to really master. In my experience, the best option is to set up a spreadsheet to help you with the process. If you are trying to figure out pricing before you even have a design, it will get even harder. With plans, you can do “take off sheets” and send the drawings out to professionals for bids.
A take off sheet is simply a place where you write down all of the materials and their costs from a specific job. You could simply send out the plans, but most professionals will give you a bid without a specific materials cost sheet. It will not do you much good to have numbers if they do not relate to anything specific. Get as absolutely detailed as you can. For example, when you are estimating the cost for the framing of the house, the obvious things to price are lumber, steel connectors, plywood, etc.. Things that can bust your estimate are all the different kinds of nails (8d, 16d, 10d, 20d, hanger and finish nails), landscape pins for the mesh, staples, etc.. These things are often thrown into the “add a few bucks for nails” column, and the results are poor estimates.
The price of steel, and thus nails and other components, is rising very quickly right now, so a “few bucks for nails” will probably not be enough. Breaking down the individual jobs like this will also help you build it in your mind, making the actual construction a bit easier later on. If you have very accurate take off sheets, you may not even need to send them to a professional for bid. You can go directly to the suppliers and get material prices. Estimating labor can be a bit more difficult without the help of professionals if you have never completed a similar job before.
After you have all the take off sheets in order, it’s time to look at labor. If you consider your own labor free, or sweat equity, great! If not, you will have to estimate how long the job will take and how much your time is worth. If you have no idea how long specific aspects will take, ask a professional (three for each trade actually). When you give a professional your take off sheets, you can ask him or her to give a cost and time estimate for the job. Consider the size of their crew, their level of experience, and then estimate your own time.
If nothing else, you may decide you don’t want to do all of the jobs you thought you would and you will have a contact and a bid from a contractor to fall back on. I have seen many people swear they want to be “really involved” in the building process and then decide that they are not quite cut out for it, or that the time commitment is adversely effecting their relationships and their own sanity!
I have seen owner/builder jobs where the cost of the house ended up at about $40 per square foot. I have also seen them get completely out of control and cost more than they would have with good consulting or if the owners had hired a contractor. The best thing to keep in mind is that it is an estimate. You can miss either way: to the conservative or to the unrealistic.
I always try to lean to the conservative side in cost estimating. If I am wrong then I come in with a price below what my client expected: pretty cool! If I am unrealistic, I loose money. If you are totally new to the idea of building, it may be worth your time to hire a consultant or work with a builder in your area on the project. Even a friend with construction experience can be valuable. If you don’t have plans yet, it will be very difficult to get an idea of price. Start with some sketches, get into the design, and estimate from there.
Eventually, you will start to recognize patterns in cost and time estimates. The estimating books I spoke of above use equations to quickly estimate cost of labor and materials. The equations are based on experience. The reason I do not like the estimate books is that the equations are based on some one else’s experience, not mine. Therefore, with every job I do, I keep detailed track of the time and budget as we build. At the end of a project, I can then take the data I compiled and add it to my own estimate sheets.
The more jobs I do, the more accurate my estimates. This is even true of jobs that cancel or I otherwise do not get. The experience of estimating improves dramatically with each experience. Practice and pay attention to the details and you will get accurate costs for your homes.