So much is heavily dependent on the individual business that a rule of thumb for a scalable data center design is both unknowable and untrustworthy. Furthermore, different companies, in even the same business sector, can have very different histories and very different expectations.
With this in mind, think about where your IT tends to operate on the "technology continuum." In other words: How often do you turn over hardware? How aggressive are you in acquiring the newest technology? How flexible are you in appropriating funds and staff to go along with the new business or technology as it occurs?
Consider all this information in the context of your knowledge of how the industry is moving. And work up actual layouts to determine the amount of space that will be realistically required over the projected time period, both for technology and for infrastructure. Considering this is not a simple task, given how hardware is getting smaller, but at the same time proliferating, and the move many companies are making toward server consolidation and virtual computing.
But no matter how confident you are with your analysis and projections, always look to locate the data center so there is opportunity for future expansion into "soft space." Soft space being area that can be taken over without having to move a department with difficult or expensive amenities and with nothing on the separating wall that would make its removal problematic, such as power panels or the communications demark. In many cases, this area is even equipped with raised floor, piping and power feeds in order to get heavy construction done at the least expense and with minimal later disruption.
The one "rule of thumb" we can provide is that it is generally accepted today that, for a major data center meeting the Uptime Institute's Tier 3 or Tier 4 standards, you should plan on the mechanical/electrical support space being at least another 50% (and probably closer to 100%) of the technology room space. In other words, whatever space you predict will be needed for cabinets and other technical hardware, double it.
ABOUT THE AUTHOR: Robert McFarlane is a pioneer in the field of building cabling design. He has been asked to speak at countless seminars on building infrastructure for electronic communications, evolving technologies and the requirements of trading floor and data center design. Mr. McFarlane served for twelve years as President of Interport Financial, Inc., a firm specializing in designs for financial trading floors and critical data centers.