Winter ADU Planning Guide: Design and Permit While You Wait for Spring

Turn Winter Waiting Into Spring Building

Every Bay Area homeowner who wants to build an ADU faces the same challenge: the best season for construction is spring and summer, but preparing for construction takes months of design, engineering, and permitting work. The homeowners who arrive at spring ready to build are the ones who used winter for preparation.

Winter planning is not busy work. It is the foundation of a successful ADU project. Every decision you make during the winter months, from floor plan selection to utility coordination to material choices, directly determines how smoothly and efficiently construction proceeds when the weather turns.

This guide walks through each step of winter ADU preparation, so you can use these months productively and arrive at spring with permits in hand and a team ready to go.

Step 1: Feasibility Assessment (December, Week 1-2)

Before investing in design and engineering, confirm that your property can support an ADU. A feasibility assessment answers the fundamental question: can you build what you want, where you want to build it?

Zoning Verification

California state law (SB 9, AB 68, and subsequent legislation) broadly allows ADUs on residential properties across the state. However, the specific rules, including maximum size, height limits, setback requirements, and parking, vary by city. Check your city’s ADU ordinance for the current regulations.

Most Bay Area cities allow:

• Detached ADUs up to 1,200 square feet on lots zoned for single-family use
• Attached ADUs up to 50% of the existing home’s floor area (with some exceptions)
• Junior ADUs (JADUs) up to 500 square feet within the existing home footprint
• Reduced setbacks (often 4 feet for rear and side yards) for ADUs

Site Evaluation

Walk your property with a tape measure or, better yet, schedule a site visit with a design professional. Identify the potential ADU location, measuring the available space and noting:

• Distance from property lines (setback compliance)
• Distance from the existing home
• Existing trees, especially protected species
• Slope and drainage patterns
• Utility locations (water meter, sewer cleanout, electrical panel)
• Access for construction equipment
• Privacy considerations for both your household and neighbors

Utility Capacity

Your ADU will need water, sewer, gas (optional if all-electric), and electricity. Assessing the capacity of your existing utility connections is an early planning step.

Water and sewer. Contact your water and sewer provider to confirm capacity. Most Bay Area utilities can serve an ADU on an existing connection, though some may require an upgraded meter or a separate connection.

Electrical. Your existing electrical panel may need an upgrade to support the ADU’s load. A 200-amp panel is typically sufficient for a home plus an ADU, but older homes with 100-amp or 125-amp panels may need an upgrade. An electrician can assess this during a site visit.

Gas. If your ADU will use gas for cooking, heating, or hot water, your gas service capacity needs to be verified. Many new Bay Area ADUs are designed as all-electric, eliminating the gas question entirely and aligning with California’s electrification goals.

Step 2: Floor Plan Design (December Through January)

With feasibility confirmed, the design process begins. ADU floor plans need to maximize livability within a compact footprint.

Common ADU Layouts

Studio (300-500 sq ft). An open floor plan with combined living, sleeping, and kitchen areas, plus a bathroom. Best for rental units, home offices with a guest suite, or aging-in-place units for family members.

One-bedroom (500-750 sq ft). A separate bedroom with a living/kitchen area and a full bathroom. The most popular ADU configuration for both rental income and multigenerational living.

Two-bedroom (750-1,200 sq ft). Two bedrooms, a living area, kitchen, and one or two bathrooms. Suitable for families or long-term housing for adult children or elderly parents.

Design Priorities for Bay Area ADUs

When designing your ADU floor plan, prioritize these elements:

Natural light. Compact spaces feel much larger with abundant natural light. Position windows to maximize daylight, and consider skylights where feasible.

Storage. Small homes need creative storage solutions. Built-in shelving, closet organizers, under-bed drawers, and kitchen pantry cabinets make a big difference in daily livability.

Indoor-outdoor connection. Even a small patio or deck off the main living area makes the ADU feel more spacious. Bay Area weather allows outdoor living for most of the year.

Privacy. Design window placement and the ADU entrance to provide privacy for both the ADU occupant and your primary residence. This is especially important for rental ADUs where the tenants are not family members.

Accessibility. Even if you are not designing for a specific mobility need, basic accessibility features (wide doorways, curbless shower, lever handles) add value and future flexibility at minimal cost.

Working with a Designer

An ADU designer or design-build firm produces the architectural drawings, coordinates engineering, and prepares the permit package. When selecting a designer, look for:

• Experience with ADU projects in your specific city
• Familiarity with current California ADU regulations
• A portfolio of completed ADU designs you can visit
• References from past ADU clients
• In-house engineering coordination (or strong relationships with engineers)

Winter is the best time to start this relationship. Designers and architects are less backlogged than in spring, and you have time for a thorough, unhurried design process.

Step 3: Engineering and Documentation (January Through February)

Once the floor plan is developed, technical professionals prepare the documents required for permit submission.

Structural Engineering

A structural engineer designs the foundation, framing system, and lateral force resistance (seismic bracing) based on the architectural plans and the soils report. The structural drawings and calculations are a required component of the permit application.

Order your soils report early in the process. It takes 2-3 weeks from boring to final report, and your structural engineer cannot begin foundation design without it.

Title 24 Energy Compliance

California requires all new construction, including ADUs, to comply with the Title 24 Building Energy Efficiency Standards. A certified energy consultant reviews the architectural plans and prepares compliance documentation showing the ADU meets insulation, window, HVAC, lighting, and solar requirements.

For ADUs, Title 24 compliance is straightforward when the design incorporates modern insulation levels, dual-pane low-E windows, efficient HVAC (heat pump or high-efficiency furnace), LED lighting, and solar readiness (or connection to the main home’s solar system).

Utility Connection Plans

Your permit application includes plans showing how the ADU connects to existing water, sewer, electrical, and gas services. This may require coordination with utility providers and, in some cases, pre-service agreements or will-serve letters.

Step 4: Permit Preparation and Submission (February Through March)

A complete ADU permit application includes:

• Architectural plans (site plan, floor plans, elevations, sections, and details)
• Structural engineering calculations and drawings
• Title 24 energy compliance documentation
• Soils report
• Utility connection plans
• Stormwater management plan
• Any city-specific supplemental forms

The quality and completeness of your application directly affects review speed. Applications missing documents or containing code errors trigger plan check corrections that add weeks to the timeline.

Submission Tips

Submit early in the week. Applications submitted Monday or Tuesday are more likely to enter the review queue that week.

Include a cover letter. A brief letter summarizing the project scope, lot information, and key design parameters helps the plan checker orient to your project quickly.

Anticipate common comments. If you know your city frequently flags certain issues (setback calculations, fire sprinkler requirements, parking), address them proactively in your plans with clear notes and dimensions.

Keep copies of everything. Maintain a complete copy of your submission package and all correspondence with the city. This becomes invaluable if questions arise during construction.

Step 5: Productive Waiting (March Through Spring)

After permit submission, the review period typically takes 6-14 weeks. Use this time for construction preparation.

Material Selection

Select all interior and exterior finishes during the waiting period:

• Exterior: Siding material, roofing, paint colors, exterior lighting
• Kitchen: Cabinetry, countertops, appliances, backsplash, sink, and faucet
• Bathroom: Tile (floor and walls), vanity, toilet, shower fixtures, mirror, and lighting
• Flooring: Material for each room (hardwood, LVP, tile, carpet)
• Interior: Paint colors, door hardware, outlet and switch covers, closet systems
• Windows and doors: Specific models and sizes from your chosen manufacturer

Having every material selected before construction starts is the single most effective way to prevent delays once building begins.

Long-Lead Item Ordering

Some items have lead times of 6-12 weeks. Order these during the permit review period so they arrive in time for installation:

• Custom windows and exterior doors
• Custom cabinetry
• Specialty tile or stone
• Appliance packages
• Custom vanities

Contractor Selection and Contracting

If you are not working with a design-build firm that handles both design and construction, use the waiting period to interview general contractors, check references, review portfolios, and negotiate contracts. Having a signed construction agreement in place before permits are approved means you can break ground immediately.

Site Preparation

While no construction can begin before permits are issued, you can prepare the site by clearing vegetation from the building footprint, removing stored items from the construction area, verifying access routes for equipment delivery, and setting up any temporary fencing required by your HOA or neighborhood.

Your Winter ADU Planning Checklist

• Verify ADU feasibility: zoning, setbacks, utility capacity (December)
• Select a design-build firm or ADU designer (December)
• Commission a topographic survey and soils report (December to January)
• Develop and finalize your floor plan (January)
• Complete structural engineering and Title 24 compliance (January to February)
• Prepare a complete permit application package (February)
• Submit permits (February to March)
• Select all materials and finishes during the permit review period (March to April)
• Order long-lead items (March to April)
• Finalize contractor agreement (if not using design-build)
• Prepare the site for construction
• Break ground when permits are approved (spring)

Why Custom Home for Your ADU

Custom Home Design and Build handles every phase of the ADU process, from initial feasibility assessment through design, permitting, and construction. Our design-build approach means your architectural designer, structural engineer, and construction team collaborate from the first meeting, producing plans that are code-compliant, cost-realistic, and construction-efficient.

We have completed ADU projects across the Bay Area and understand the specific requirements of cities throughout Santa Clara County, San Mateo County, and Alameda County. Our permit applications are thorough and address common plan check issues proactively, reducing review times and revision cycles.

Our Phase 1 design process covers everything from programming through permit-ready documents, so you enter construction with confidence that every detail has been addressed.

Start Planning Your ADU This Winter

The ADU you want to build next spring starts with the planning you do this winter. Every week invested in design, engineering, and permit preparation now translates to a smoother, faster construction process when the weather cooperates.

Contact Custom Home Design and Build to schedule an ADU feasibility assessment. We will evaluate your property, discuss your goals, and map out a winter planning timeline that positions you for a spring ground-breaking.