It's easy. For the past fifteen years we have used an all-inclusive template for documenting our estimates of project costs. The first embedded template shows our format. This part summarizes every cost that we think the client will face. The construction costs are pretty obvious, and even the breakdown isn't very unusual.
However even during Pre-Design we are able to create this level of detail. The reason we can do that is the second embedded document - the assumptions and the quantities we derive from those assumptions. From there we feed the quantities into an RS Means CostWorks model for the construction type that we anticipate. The result is surprisingly accurate.
Having created a project cost projection, it is important to live within the assumptions that you have made. Very few architects do this and our reputation regarding cost control is a result. There are all kinds of weasel words to hide behind, but you don't see contractors shying away. It's not FM. It takes a little time, but you recoup it on every estimate you do thereafter.
The secret sauce is the quantity takeoff without drawings. This is done once and rarely needs to change. Special features just become new line items. You set up formulas in your spreadsheet to calculate every element of construction that you have. Many of these are very simple. For instance, the quantity of flooring usually equals the net area of the building. The amount of carpet flooring is simply the net area times the percent of the floors you think will be carpet. The quantity takeoff spreadsheet refers to the assumptions that you have documented on the assumption sheet of the workbook.
Another example is windows. This is more complicated because it works with assumptions about gross area, building proportions, number of stories, story height, and percent of exterior wall that is glass. The trickiest calculations are square feet of interior partitions and number of doors. These require factors pulled from a table. The factors can be found with some digging. If you work with just a few building types, you can develop your own from past projects.
Once the data is entered, we have all the quantities. The last step is to transfer the quantities to the model you built in Means CostWorks. With a little forethought this is a simple transcribing exercise that takes twenty minutes. When you have your system set up, which will take about three man-days, an estimate can be pulled together in an hour, including the twenty minutes of transcription. You are probably spending more time than that and getting poorer results.