It's a good idea when you have a plot of land and a set of plans to look at what costing's you are likely to incur in your Build, a simple idea is to break down the the build into various stages or categories such as below.
Each chunk or stage of a build can be costed by sending out plans and relevant information (Tender Packs) to contractors and builders to quote or tender for the work to be done, it is a good idea to get at least 3 quotes or tenders for each stage, it's worth mentioning that it's not always the cheapest quote that should get the job, a selection process should be used to pick the contractor that can fit to your time table and include all aspects of work you have set out in the Tender pack. A simple contract can be made between the 2 parties (you and the Contractor) and an agreed payment schedule based on completed work.
Electrical & Mechanical, Commonly know as M&E
Do you need an M&E Contractor? It Depends on the size of your development, for a House with a Total floor area in excess of 2000 square feet I feel You should be looking at M&E in a slightly different way to a smaller development.
For Example (M&E) A smaller development may adequately use for Ventilation local extraction fans in rooms extracting air through an external wall A Larger Development may require low noise Ventilation to all rooms with discreet hidden Ducting with heat recover incorporated to contribute towards Building Energy Management. Heating design will be different systems for small and large developments, Hot and cold water supply "runs" around the house will increase in complexity as a project size gets bigger (more floor space to heat and longer runs from Boilers to taps). Larger Projects tend to have extra storage tanks, pumps, larger capacity pipes, buffer tanks and more in terms of Electronic Home Control systems as seen in:- for a more detailed Excel spreadsheet free Download,
In the Free Download Example we have indicated the kind of amounts you would possibly spend for each stage on an average sized 4 bed detached house, we must point out this is only an example and is now 3/4 years old, however it is not far from being a reasonable benchmark to use as a starting point to estimate your build (these figures are based on a recent House build QS I recently prepared for a client) for example:-