Design for Today's Problems, Not Tomorrow's

p. 110 of Extreme Programming.

"The traditional strategy for reducing the cost of software over time is to reduce the probability and cost of rework. XP goes exactly backwards. Rather than reduce the frequency of rework, XP revels in rework...
"The key is that risk is money just as much as time is money. If you put in a design feature today and you use it tomorrow, you win,because you pay less to put it in today.... However... If there is enough uncertainty, the value of the option of waiting is high enough that you would be better off waiting.
"Design isn't free. another aspect of this situation is that when you put more design in today, you increase the overhead of the system. There is more to test, more to understand, more to explain. So every day you don't just pay interest on the money you spent, you also pay a little design tax. With this in mind, the difference in today's investment and tomorrow's investment can be much greater and it is still a good idea to design tomorrow for tomorrow's problems.
"As if this weren't enough, the killer is risk.... You can't just evaluate the cost of something that happens tomorrow. You also have to evaluate the probability of it happening....
"So, the cost of making a design decision today is the cost of making the decision plus the interest we pay on that money plus the inertia it adds to the system [emph. added]. The benefit of making a design decision today is the expected value of the decision being profitably used in the future.
"If the cost of today's decision is high, and the probability of its being right is low, and the probability of knowing a better way tomorrow is high, and the cost of putting in the design tomorrow is low, then we can conclude that we should never make a design decision today if we don't need it today."

Example: From Chapter 3
"Suppose you're programming merrily along and you see that you could add a feature that would cost you $10. you figure the ROI on this feature (its net present value or NPV) is somewhere around $15. So the NPV of adding this feature is $5.
"Suppose you knew in your heart that it wasn't clear at all how much this new feature would be worth -- it was just your guess, not something you really knew was worth $15 to the customer. In fact, you figure that its value to the customer could vary as much as 100% from your estimate. Suppose further that it would still cost you about $10 to add that feature one year from now.
"What would be the value of the strategy of just waiting, of not implementing the feature now? Well, at the usual interest rates of about 5%, the options theory calculator cranks out a value of $7.87.
"The option of waiting is worth more than the value (NPV = $5) of investing now to add the feature. Why? With that much uncertainty, the feature certainly might be much more valuable to the customer, in which case you're no worse off waiting than you would have been by implementing it now. Or it could be worth zilch--in which case you've saved the trouble of a worthless exercise.
"In the jargon of trading, options 'eliminate downside risk'".