My own work is on urban green space, so my comments on the question at hand are oriented toward that context. Understanding “the political” in terms of general democratic accessibility, three limits of design come to mind.
First, designs will be limited based on what designers can know or imagine about the conventions of a space’s use. Designers asked to design for politics must come up with some definition of what good politics looks like—and the spaces they produce will inevitably reflect designers’ particular biases and normative assumptions in ways that directly affect the inclusivity of those spaces. For instance, parks and urban public green tend to reflect quite culturally and historically specific assumptions about what forms of engagement with the environment are desirable or appropriate, and what kinds of activities and social interactions are good to cultivate through design. (Though of course relatively “open” designs are less subject to this.)
Second, design can’t address the unequal outcomes and adverse consequences of the broader political and economic context in which projects take place. For example, a paradox of urban greening projects is that efforts to increase the liveability of urban areas by adding green public space also make them more desirable and therefore more expensive—often displacing those who stood to benefit the most from improvements, who are also frequently those who advocated for the improvements in the first place. The result is that design-based efforts to improve democratic accessibility within a certain space may make the broader geographic area (the neighbourhood surrounding a park, for example) less inclusive as they contribute to making them more expensive. This is not a problem of design but of geopolitical context, and accordingly tools to combat it lie outside of design itself, such as rent regulation or political activism.
Third, thinking about design for democracy also begs the question of how to democratize design (thanks to Theatrum Mundi friend and fellow participant, Gianpaolo Baiocchi, for this point). How can we democratize the design process itself, not only to create good designs but a process that is responsive to non-expert views on what good design is? The many arguments against “participatory design” suggest that we haven’t yet figured out how to democratize this domain. Tokenistic or legislated forms of participation have become substitutes for truly inclusive processes. Laypeople are ill-equipped to give designers information they can translate into realistic and affordable plans. In community-based processes, the most well-organized and well-resourced constituencies tend to have the loudest voices, and may or may not represent the interests of the whole. In terms of design outcomes, tastes differ and possibilities are constrained. So what does a successful democratic design process look like?
This all sounds very dark, but I do not mean to suggest that design does not have an important role to play in politics, nor even that these are limitations that can or should be overcome. Rather, these are common and predictable patterns and problems that we might just keep in mind when undertaking any self-consciously political design or design process.