Building a modular home (also known as prefab construction) on an urban infill lot requires a very different approach than building on a larger suburban parcel – especially in California, where stricter building codes and evolving housing laws can impact design, approvals, and construction.
When we began designing our dream home, we didn’t think much about how the size of the lot might affect the project. With experience rehabbing homes, we assumed that as long as we understood zoning, setbacks, and height limits, we could build from the ground up without major issues.
That assumption didn’t last long.
As we began working with the Planning Department and designing our home, challenges started to surface – some we didn’t anticipate, and some the modular builders we worked with hadn’t encountered either.
On a small urban infill lot, these limitations ended up shaping nearly every design decision – and adding costs we hadn’t planned for.
“You Can Build Anything You Want”
When we first went to the Planning Department, we were told we would need to submit for a Certificate of Compliance to confirm the parcel was a legally buildable lot. During that meeting, we were also told the property sat within a planned development zoning district, and that this type of infill zoning would provide flexibility in how a single-family home could be designed.
Planned development is often used in infill housing development to allow projects that respond to surrounding conditions, giving developers more flexibility in land use and design.
We were even told, “You can build anything you want.”
That turned out not to be the case.
Once we submitted drawings for our home, the city required us to follow standard residential zoning rules – along with additional limitations that weren’t clearly documented in the code.
We go into more detail in this post, where we share what we learned navigating the planning process, zoning challenges, and approval hurdles before applying for a building permit.
Takeaway: Even when you perform due diligence by reviewing zoning ordinances early in the planning phase, urban infill projects can still face changing requirements and unexpected constraints – even after completing streamlined feasibility studies.
Above-Grade Foundations on Small Lots: Design and Construction Impacts
Urban infill lots are typically small parcels in established neighborhoods where surrounding properties have already been developed. Our lot is less than 2,500 square feet, with neighboring homes built close to the property lines.
Every decision had to fit within tight limitations – balancing the building footprint, setbacks, driveway access, and usable outdoor areas within a very limited footprint. On underutilized urban lots like ours, maximizing usable area becomes critical to making the design work.
What initially seemed straightforward quickly became a series of design adjustments driven by layout limitations and the realities of building on a small urban lot.
Most modular homes are designed to sit on foundations that extend above grade.
On larger suburban lots, this isn’t very noticeable. On a small urban lot, however, foundation height can create real design tradeoffs.
In our case:
- Increased building height pushed us closer to allowable limits
- The home appeared taller than neighboring houses during city review
- Additional entry steps extended into already limited outdoor areas
Each entry step typically extends out roughly 10–11 inches, and the required landing outside an exterior door must be at least 36 inches deep. On our narrow lot, even one additional step would have pushed into the driveway at one entry and into the small rear patio.
To address this, the factory designed a fully below-grade basement. This allowed the home to sit lower relative to the street, reduced the number of steps, and better matched the scale of surrounding homes. This design required precise alignment between the main floor modules and the foundation walls.
Excavation and Site Work on Small Urban Lots
Just as we addressed height and space constraints, another issue surfaced.
Building a basement on a tight lot requires additional engineering due to depth and the proximity of neighboring structures. The city required a geotechnical (soils) engineer to evaluate site conditions.
The soils report determined that engineered shoring would be required to support the surrounding soil before work could begin.
This added another layer of planning, coordination, and cost that we hadn’t anticipated. We’ll share more in future articles – including the excavation and shoring process, and how we integrated the modular home with the basement foundation to make it work on a constrained site.
California Housing Regulations Impacting Urban Infill Development
Our project also had to adapt mid-stream to new housing regulations aimed at increasing density on residential lots in California. These policies are part of broader efforts to address housing shortages and better utilize existing residential land. These requirements often include adding an Accessory Dwelling Unit (ADU) or Junior ADU (JADU), even on smaller properties.
On a constrained lot, these requirements introduced additional design and planning requirements. Many of these regulations were new – not just to us, but also to local jurisdictions – which led to confusion and inconsistent interpretations during the approval process.
In hindsight, starting this research earlier would have helped us eliminate unnecessary costs and achieve significant time savings during planning.
Pro Tip: Visit our post, “The Real Cost of a Modular Home,” to understand the questions to ask builders early – including how fees, taxes, and project scope can vary.
Can You Build a Modular Home on an Urban Infill Lot?
Yes – you can build on an urban infill lot, but it comes with added complexity compared to larger sites.
Tight lot dimensions, foundation height, excavation depth, and limited construction access all introduce challenges that many buyers don’t anticipate. These constraints often require design adjustments and additional engineering compared to building on a larger suburban lot.
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