It’s easy to get excited about a floor plan before you’ve confirmed anything about your actual lot. It’s also the most common mistake we see. Redfin recently published a guide on ADU planning and asked the Apex Homes team to weigh in, along with a handful of other builders and ADU experts. It got us thinking about the questions homeowners should really be asking before they start designing — not after.
Start With Your City, Not Your Design
As Redfin’s guide points out, ADU rules aren’t uniform. Setbacks, unit size limits, parking requirements, and owner-occupancy rules can vary from one city to the next — sometimes even block to block within the same city. What’s allowed in Sunnyvale won’t necessarily fly in Los Gatos or Mountain View. Before you commit to a layout, confirm what your specific address allows.
Feasibility Comes First — Always
This is the point we made directly in Redfin’s piece: homeowners should always start with a feasibility study to understand what is actually possible on their lot, including zoning, setbacks, utility access, permitting requirements, and budget expectations.
Skip this step and you risk falling for a design your lot can’t actually support — then redesigning from scratch, or worse, finding out mid-permit that the plan doesn’t work.
Budget Beyond the Build
Redfin’s guide also flags “soft costs” that can add tens of thousands of dollars before a single wall goes up. Homeowners should budget for:
- Permits and impact fees
- Utility connections (water, sewer, gas, electric)
- Engineering and site surveys
- Design and architectural fees
- A contingency fund for surprises — especially on older lots or aging utility infrastructure
This is part of why we build solar, appliances, and full permit handling into our pricing upfront, rather than leaving these costs to surface mid-project.
Decide How You’ll Use It Before You Design It
A rental unit, an in-law suite, and a home office all call for different layouts, privacy needs, and utility connections — another point Redfin’s guide raises. Deciding the ADU’s purpose early avoids expensive retrofits later, since accessibility features, private entrances, and sound insulation are far cheaper to plan for than to add after the fact.
Pre-Engineered Plans Can Help — With Caveats
Redfin’s guide notes that pre-approved plans can speed up the design phase, but “pre-approved” doesn’t automatically mean permit-ready everywhere. Most cities still require site-specific reviews for utilities, foundation, and grading. That’s why our pre-approved floor plans are matched to the specific cities we build in, rather than a generic catalog.
Read Redfin’s full guide here: What to Know Before Adding an ADU
Find Out What’s Possible on Your Lot
The fastest way to answer most of these questions is a real feasibility check. Our free ParcelZ tool shows you what’s buildable on your address in minutes — before you spend time or money on a design.