The trend of urbanization in today’s cities complicates the challenges relating to access to housing and poses threats to the well-being of many urban residents. Worldwide, governments often resort to affordable housing as one of the several approaches to providing adequate housing units for their city inhabitants [1,2]. In the literature, affordable housing is often described by its main supply motive of improving housing affordability, often among low-to-middle-income households who are limited in their ability to compete in the mainstream housing market [3,4,5,6]. In most cases, this category of target households falls within the bottom 40% of the income distribution spectrum within the city [7,8]. The common measure of housing affordability is that occupants of a given household should ideally spend no more than 30% of their income on housing costs, such that other basic non-housing needs can be afforded [3,6]. The price-to-income index is criticized as overly focused on economic intentions [8,9,10], yet it still dominates planning decisions today as it allows attention to be paid to the acquisition and operation cost barriers that many low-to-middle-income households face in the housing market [11,12].

With the advent of sustainability thinking toward the end of the 1980s, scholars argue that the science and policy of affordable housing should extend beyond price-to-income affordability assessments to include the extent to which the housing delivery process and its outcome respond to the broader needs of the occupants [13,14]. The concerns stem from the fact that occupants of affordable housing units, like any other urban dweller, require access to health services, good access roads, well-developed urban green spaces, and cost-effective and environmentally friendly utilities, among others [13,15,16,17,18]. Embracing sustainability therefore implies adopting a need-based approach to affordable housing design and delivery. The question regarding what constitutes human needs, especially within the housing discourse, has been an area of contention given its subjectivity. For example, Ytrehus, in a critical discussion of needs within the housing discourse, identified both physical and spatial dimensions of housing needs [19]. A similar observation is seen in the argument that housing needs extend beyond physical space to include cultural, socioeconomic, environmental, and functional requirements [20].

Scholars have therefore begun substituting “Affordable Housing” in the literature with “Sustainable Affordable Housing (SAH)” [17,18,21,22,23,24,25]. Ref. [23] describe SAH as one that maximizes occupant freedom, promotes quality of life, and is responsive to social issues such as security, cohesion, and privacy while contributing toward reducing stress, among others. Ref. [18] similarly observed that SAH should facilitate limited resource use, for example, energy, water, and automobile reliance. The needs and demands of occupants of affordable housing are therefore multifaceted and can be analyzed in tandem with the broader dimensions of sustainability: economic, environmental, and social [14,26,27,28]. In this case and for this research, affordable housing becomes sustainable when it integrates important variables in the housing web such as housing and transport costs (acquisition and operating costs), household satisfaction (determined by user satisfaction with the use of the unit and its accompanying facilities, the functionality of the unit, and the unit’s safety performance), and the efficiency of stakeholder management (neighborhood or stakeholder satisfaction and low prevalence of disputes) [6]. This conceptualization is seen as broad and sensitive to developing contexts, hence its adoption for this study.

With human needs being at the core of the SAH discourse, the critical question here is how can the process and outcome of affordable housing delivery, especially among low-to-middle-income households, be organized to respond to the needs of the occupants such that their overall wellness is improved? In principle, it is difficult to present an objective conceptualization of wellness in the SAH discourse, which this study assumes to be synonymous with “well-being”. The Stanford Research Institute suggested that the World Health Organization’s definition of the concept of “health”—the physical, psychological, and social well-being of an individual—can be taken to broadly embody the tenets of wellness [29]. Many built environment scholars adopt a pragmatic approach in analyzing occupant wellness, often as a measure of stress [30,31], estimated by the influence of the nature of the housing environment, both indoor and outdoor elements, on the lifestyle adjustments and comfort levels of the occupants, including their physical body’s functioning, mobility, and general ease of life [32,33,34,35]. Scholars such as [36] for example, dwelt on occupants’ experiences regarding a positive sense of self and security in estimating their wellness. The approach is consistent with others who analyzed wellness as a function of the level of occupant satisfaction with a combination of several indoor and outdoor attributes of the housing unit. Such studies assume the form of either a combined or isolated assessment of indoor environmental quality [37], indoor air quality [38], or outdoor environmental quality [39] among others. Ref. [40] assessed the level of wellness among occupants of low-cost housing units in Malaysia based on the occupant’s level of satisfaction across four categories of housing features: the housing design and space arrangements, the level of indoor comfort, green areas, the landscape and recreational area, and the service area. The observed flexibility in the selection and inclusion of variables for the assessment of occupant wellness therefore suggests that how housing and its environment are configured has a direct impact on the lifestyle adjustments made by occupants and must be directed by appropriate local planning policies and tools.

In many cities today, governments tend to shift toward user-centric philosophies that optimize housing neighborhoods given the relative scarcity of urban land and other resources that are bottlenecks to delivering sustainable, affordable housing [32,41,42,43,44]. Such density optimization ideologies align with the tenets of the Geddesian Triad for neighborhood planning [45,46], which seeks to facilitate an optimal nexus between people and housing densities, such that the housing product in a way that the housing unit and its environment will respond positively to contextual sustainability goals. In many cases, the application of this ideology to solving housing problems in cities is implemented within the framework of urban transformations and not necessarily through the creation of new cities [47]. More explicitly, Gordon’s work in 2008 and his later submissions with Whitehead demonstrated how optimization of densities in physical planning can help attain sustainable housing outcomes even if planners’ control over how they evolve is limited in many cases [48,49]. Other scholars showed the potential of frameworks such as Metropolitan Green Belts tenets in contributing toward managing neighborhoods sustainably [50]. Despite the many theorized approaches, only a few scholars provide what can be described as practical conceptual models for translating the ideologies into actual affordable housing and neighborhood configuration solutions [17]. The few that sought to mitigate this gap proposed innovative pattern languages for designing rapidly urbanizing regions, including the proposition of biophilic urbanism design patterns that optimize the interconnectedness of occupants to their natural environment or level city patterns that control building heights toward the minimization of the negative environmental, economic, social, and psychological effects of poorly managed densities, among others [51]. The emergence of innovative design ideologies that leverage the tenets of the New Urban Agenda to propose patterns for neighborhood configuration toward optimizing space benefits in urban planning and design and toward the realization of broader sustainability goals is thus both timely and less contested in relevance. Thus, the purpose of this study is to provide an alternative conceptual model that can help tailor affordable housing to the needs and wants of occupants, particularly in developing countries.

Within this context, the aims of this entry are as follows: a. to conceptualize the interconnectedness of three important variables for SAH planning and design: affordability, density, and occupant wellness, toward the development of a new philosophical model, and b. to provide an alternative conceptual tool that leverages the potentials of the traditional vertical subdivision process of land for affordable housing delivery that is based on the contextual needs of occupants and directed toward the realization of broader sustainable housing outcomes. This entry delimits the focus of the first objective to the initial conceptualization of the interconnectedness between affordability, density, and occupant wellness. The outcome, which is depicted in the conceptual framework (see Section 4), is planned for further development of the advanced philosophical model in a series of planned manuscripts constituting an ongoing doctoral thesis on the development of a new planning model for SAH at the University of Melbourne, that maximizes occupant wellness in the affordable housing delivery process. To implement these objectives, a review of literature from the Web of Science and Google Scholar databases was conducted between March 2023 and August 2023. The two databases were selected due to their wide coverage of high-influence inter-disciplinary journal publications and other relevant policy documents on the subject of interest. The search included the following keywords: affordable housing, vertical subdivision, density, and occupant wellness. The keywords were combined using the Boolean operators “AND” and “OR”. Due to the interdisciplinary nature of the objectives of this study, the resultant publications and policy documents were refined manually on an incremental basis according to their relevance to the two main research objectives. Only publications in peer-reviewed journals and policy documents from reputable organizations such as the UN-Habitat were included until a point of saturation (perceived sufficiency of existing evidence based on authors’ discretion and familiarity with the field) was reached. The adapted approach varies slightly from existing approaches for conducting a literature review (for example, the PRISMA approach [52], as the paper constitutes a conceptual work and not a traditional systematic review paper). All sources included in the study are outlined in the list of references.

The research was driven by the fundamental assumption that if housing and its neighborhoods can be configured under optimal densities in a way that allows low-to-middle-income households to meet their changing needs, with the flexibility to alter the housing unit to the changing needs of each household, lifestyle adjustments for comfort are likely to reduce, thereby increasing the overall wellness of the occupants.

This entry is organized into five main parts. The second part discusses the common approaches to affordable housing delivery, with a focus on developing contexts. The third part describes the theoretical underpinnings of compact housing planning, which then informs our conceptualization of the vertical subdivision development process for affordable housing delivery in the fourth part. The authors further presented a framework for the application of vertical subdivision development in the same part. In the fifth and final part, the authors conclude their main arguments in the entry and present an outlook of potential research based on the new paradigm.