This is a myth we are happy to dispel.
The short answer is no, it doesn't need to be any more expensive to design a sustainable building than it costs to build a standard code compliant building. There are so many tools in our toolbox that just require more careful thought - substituting certain materials (often at the same cost), orienting your building to maximize solar gain in winter, while keeping the hot sun out in summer; replacing standard toilets and basin taps with low water usage fittings (again at minimal cost), incorporating thermal mass elements to store heat and regulate temperature fluctuations...the list goes on.
When we start adding imported extras such as solar panels, ground source heat pumps, triple glazed windows etc., then, yes, there can be as much as a 10-15% premium for your build. You may well be willing to pay more in your build costs up front to save money in your running costs over the life of your building, and many clients choose this option.
It really all depends how much you can afford to do. Even if you are on a tight budget, don't rule out sustainable design completely. While you may not have the budget for all of the bells and whistles in terms of energy savings, you can at least afford a home or office space that is healthy (non-toxic), warm and dry. And you can still do your bit for the environment.
We work closely with professional quantity surveyors to provide you with a "shopping list" of options to choose from when designing your new space.
As with everything, doing something is better than doing nothing. Let's do what we can.