Most things in life don't happen all at once, but instead build over time. And perhaps like the compound theory of interest — the same idea that is used to build wealth over time — there might be a compound theory of architecture or urban design: an idea that our built environment is best when it accretes over time.
We don't often think this way explicitly about architecture. It remains too easy to think of any architectural act as an independent event. When we do think of the larger context, it is most often to try and understand how any new object will interact with its surroundings. But this still treats architecture as an independent object, one that acts on or is acted upon. Thinking accretively implies more than this superficial reading of context, by reminding us that there is an opportunity cost inherent in every change to our built environment.
Opportunity cost is an economic term, but an intensely practical one. It is the calculation of that which could be done instead. There are benefits that are given up by choosing any one action over another, so weighing the benefits and costs of all potential options and then comparing them helps lead to better choices. It is as if we imagine a bunch of parallel worlds where all different possibilities are implemented, and we then choose which one gives us the best outcome.
The trick, however, is not to think of opportunity costs in strictly economic terms. Opportunity costs also exist culturally. Every architectural intervention re-"places" something. It prioritizes something, it eliminates something else. Keeping an eye on the power of accretion helps refocus those decisions. It ensures that when we prioritize something, that we truly do value it and it is not just expedient, and that when we eliminate something, it is not something that enriches us but something which we can live without.
I believe the power of accretion is understood intuitively. We know that eliminating waste and rework will get us to our goals more expediently, and that frivolously discarding and replacing is a waste of resources. The compound part happens when we are effective at this. The whole often adds up to much more than the individual parts.
In architectural discourse, I have sometimes heard the word "accretion" used to justify when a proposal doesn't feel right — when a designer is perhaps too ambitious or too insensitive or trying to do too much. However I have rarely, if ever, seen it used as a guiding principle. This is a shame, because understanding and valuing accretion can help address many situations that I have been discussing in earlier posts.
First, accretion addresses the conflict between history and development. (see The Encapsulation of Values) If we think accretively, we are loathe to destroy anything of value that already exists. It is not only wasteful, but actually scarring -- we would be better off having two things of value, one new and one old, rather than just one new thing.
Second, accretion switched the primary focus of sustainability from building sustainably — which is still building after all, and adding to our resource usage — to simple preservation and maintenance. (see Sustainability Has a Branding Problem) It admits that creation is important, and that we will have new things added to the world with each and every generation. We should continue to do this as sustainably as possible and with a long term focus. But thinking accretively also helps, by ensuring we are not wasteful or disregarding the value in the existing built fabric. Sustainability is not just about building with sustainable materials or using less energy, it should also be about building smarter, less often, and with more durability.
Third, accretion reduces ego and introduces an element of humility. (see The Multitude of Creation) It is easy for designers to get all too carried away given the opportunity. Ensuring that plans are introduced into a context of accretion should help guard against overreach. Accretion is not only about being sensitive to the history of what exists, but also leaves room for future generations to contribute to the cultural fabric.
Fourth, accretion promotes a diverse environment. (see The Need to Create) At its heart, this approach attempts to provide space for a wide variety of creators, expressions, and voices. With a more diverse picture, a more accurate version of a culture is apt to arise.
Accretion in architecture should not be taken too literally, as a way of thinking of an architectural act. It does not mean we should only build additions or never touch the existing building stock. Thinking accretively should instead steer us to thinking more sensitively of the sites and programs we choose, how much we invest in them, and whether we should initiate a project at any point in time.
Though accretion may not quite tell us what to build, it does, however, give us a clear moral direction for that which we should not do. This is actually quite important, particularly at this moment in time, when it seems like anything is possible and permitted.
In my very first post on this site, I offered a checklist when thinking about an approach to a project. I still find it quite relevant.
Don't do anything.
Find what is already good and keep it.
Remove things first.
Add sparingly.
And maximize the compound power of accretion.