What's the number one cost in web design? Is it planning the application? Is it building the application for the first time? Is it the cost of the server or the connectivity?
Actually, it's none of these things. The highest cost will be maintaining the application. This is in contrast to traditional systems design, where a majority of expense is in the systems understanding and analysis.
Why the difference? With traditional systems, an application has a typical lifetime of 2 to 4 years. Once the application has reached a stable configuration the design effort is all but finished.
On the other hand, the dynamic aspect of the web makes most applications outdated in months rather than years. Improvements in existing applications are continuous, new content is expected, new technologies appear on a regular basis. A web application must improvise or be replaced. Another way to put it is that web software is in a continual prototyping process - user feedback and design demands are much more prevalent than in their traditional counterparts.
The fact remains that your software must be frequently modified and updated. Realizing this at an early point in the design phase is crucial to making maintenance easier in the future. Trust me, I've learned the hard way.
Build customization variables into your code. Separate code and content. Use templates. Use style sheets. Make the language of your site customizable. Develop a common coding style and stick with it. Comment your code well.
To put it simply, you can do what we do, and build everything for distribution. It may seem like an unnecessary investment of time and resources if you a designing for one customer only, but, trust me, when the client starts asking you to make changes.