Seasonal ADU Care for San Francisco: Year-Round Homeowner's Guide

Seasonal ADU Care for San Francisco: Year-Round Homeowner’s Guide

October in San Francisco is not fall. It’s the hottest month of the year, and it’s also when the first serious rain events roll in off the Pacific. That means two completely opposite maintenance tasks, cooling system checks and drainage prep, are due in the same 30-day window. If you own an ADU in San Francisco, whether it’s a backyard cottage in the Sunset or a garage conversion in Bernal Heights, a generic national maintenance calendar will steer you wrong every single time. This guide rebuilds the ADU care calendar around how weather actually behaves here in San Francisco, season by season, with honest time estimates and cost ranges so you can plan around tenants, rental income, and your own sanity.

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Quick Answer

San Francisco ADU care should be organized around four real seasons: the wet season (October through March), the fog season (April through June), the fire-weather window (July through September), and the deceptive “second summer” stretch in September and October. Focus most of your effort on drainage, envelope sealing, and mini-split maintenance, because those are the three systems that fail most often in San Francisco’s specific climate. Budget roughly 8 to 12 hours of hands-on maintenance per year, plus $300 to $900 annually for professional inspections and service.

Table of Contents

• Wet Season Prep: October Through March
• Roof and Deck Drainage: The Flat-Roof Problem Most Guides Ignore
• Fog Season Envelope Checks: April Through June
• Fire-Weather Window: July Through September
• Mini-Split Maintenance: What Title 24 Requires and When to Call
• Tenant Communication Templates for Every Season
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions

Wet Season Prep: October Through March

The wet season is when San Francisco ADUs take the most damage, and it’s not because of dramatic storms. It’s because moisture sits. For five straight months, fog drip, light rain, and high humidity keep surfaces damp. Wood swells. Caulk fails slowly. Fasteners loosen. By February, you have rot, mildew, or a slow leak that’s been quietly degrading a corner of the ADU for weeks before anyone notices. The fix is front-loaded: spend 3 to 4 hours in early October getting the envelope tight, then do 30-minute spot checks after each significant rain event.

October: The Critical Month

October is when the second summer heat is still lingering AND the first real rain arrives. You need to do two opposing things at once: confirm the cooling systems still work, and prep the drainage systems for water. This is the single most important maintenance month of the year for a San Francisco ADU.

1. Clean all gutters and downspouts (90 minutes). San Francisco’s eucalyptus trees, pines, and cypresses drop needles and bark constantly. A clogged downspout turns a normal rain into a waterfall against your ADU’s siding. If your ADU has a flat roof, this step is non-negotiable.
2. Inspect all caulking and weatherstripping (45 minutes). Walk the perimeter. Check window frames, door thresholds, and any penetration where a pipe, conduit, or vent exits the wall. Silicone caulk lasts 3 to 5 years in San Francisco’s UV and salt air; if it’s cracking or pulling away, scrape and recaulk now.
3. Test the sump pump, if you have one (15 minutes). Pour a bucket of water into the pit and confirm the pump kicks on. A dead sump pump discovered during an atmospheric river is a flooded ADU.
4. Check the water heater pressure relief valve (10 minutes). Lift the lever briefly. If water doesn’t come out, the valve is stuck and needs replacement before the cold season increases demand.

If you’re adding an ADU or planning a home addition in San Francisco, insist on a drainage plan that accounts for San Francisco’s peculiar rain pattern: long, light drizzle rather than heavy downpours. Roof slopes of less than 1/4 inch per foot will pond water. That’s a design flaw, not a maintenance issue, and it should be caught before construction starts.

November Through March: The Spot-Check Routine

Once October prep is done, the wet season maintenance burden drops significantly. Set a recurring 30-minute check after each rain event of more than half an inch. Look for water staining on ceilings, damp spots on walls near windows, and pooling water against the foundation. Address anything you find within 48 hours. In our experience with ADUs across San Francisco, most water damage that becomes a $5,000 repair started as a $50 caulk job that got skipped in November.

Cost range for wet-season prep: $0 if you do it yourself with a caulk gun and ladder, or $150 to $400 if you hire a handyman to do the full October checklist. That’s cheap insurance compared to the $3,000 to $8,000 typical cost of repairing water-damaged drywall, insulation, and flooring in an ADU.

Roof and Deck Drainage: The Flat-Roof Problem Most Guides Ignore

Here’s a fact most national ADU maintenance guides miss entirely: a large percentage of San Francisco ADUs have flat or low-slope roofs, and many have rooftop decks. Pitched-roof guides tell you to check shingles. That advice is useless for a Sunset District ADU with a TPO membrane roof or a Mission District unit with a fiberglass deck over living space. Flat roofs fail at the seams, the drains, and the flashing, not the field of the roof itself. And rooftop decks, which are common on San Francisco ADUs because they capture views and outdoor space, are notorious for slow leaks that travel horizontally before showing up inside.

The 4-Step Flat Roof and Deck Inspection

1. Clear the drains and scuppers (20 minutes). Flat roofs drain through internal drains or edge scuppers, not gutters. A single blocked drain turns the entire roof into a pond. Remove leaves, pine needles, and any debris. In the Richmond and Sunset districts, wind-blown sand can accumulate and clog drains surprisingly fast.
2. Inspect membrane seams (30 minutes). Walk the roof carefully (or use a drone if you can’t safely access it). Look for bubbles, cracks, exposed fasteners, or areas where the membrane has pulled away from penetrations like vents and skylights. In San Francisco’s fog, membrane seams experience constant wet-dry cycling from June through August, which accelerates adhesive breakdown.
3. Check deck pedestal and waterproofing layer (45 minutes if you have a deck). If your rooftop deck sits on pavers or tiles over a pedestal system, the waterproofing layer underneath is the real roof. Pull up one or two pavers in different locations and check for standing water, cracks, or debris accumulation. If the pedestals are sitting in water, the drainage below is failing, even if the deck surface looks fine.
4. Inspect flashing at walls and parapets (20 minutes). The junction where the roof meets a wall or parapet is the most common leak point on San Francisco flat roofs. Look for cracked sealant, rusted metal flashing, or gaps. Re-seal with a compatible sealant, or call a professional if the flashing shows signs of rust-through.

Honest time estimate: a full flat-roof and deck drainage inspection takes 2 to 3 hours twice a year, once in October before the rains and once in April after the wet season ends. If you’re not comfortable walking a roof, hire a professional. A roof inspection in San Francisco typically costs $200 to $500, and a drone-based inspection is on the lower end. When we build ADUs in San Francisco, we design drainage redundancies specifically for the flat-roof reality: secondary scuppers, sloped underlayment, and deck systems that don’t trap water against the membrane.

Fog Season Envelope Checks: April Through June

April through June in San Francisco is fog season. In the western neighborhoods, the Outer Sunset, the Richmond, Parkside, and the Presidio, the fog can sit for days. Relative humidity stays high. Surfaces that never see direct sun don’t dry out. This is when you discover whether your ADU’s envelope, the exterior skin that separates inside from outside, is actually doing its job. Fog season is also the best time to test for air leaks, because the temperature difference between inside and outside makes drafts obvious.

What to Check During Fog Season

• Window and door seals (30 minutes). On a foggy morning, walk the interior perimeter of the ADU and feel for cool air moving around window and door frames. A dollar bill test works: close the door or window on a dollar bill and try to pull it out. If it slides out easily, the seal is too loose.
• Ventilation and exhaust fans (20 minutes). Bathroom and kitchen exhaust fans need to move moist air out. Turn each fan on and hold a tissue near the vent. If the tissue doesn’t stick, the fan is weak or the duct is blocked. In San Francisco, a poorly ventilated bathroom turns into a mildew farm by July.
• Under-floor crawlspace and vents (30 minutes if accessible). Many San Francisco ADUs sit on raised foundations with crawlspaces. During fog season, crawlspace vents that were open all winter have been letting damp air into the structure. Check for standing water, visible mold, or musty odors. If you see any, close the vents or add a vapor barrier.
• Deck and railing fasteners (30 minutes). The salt air and moisture in San Francisco’s western neighborhoods corrode exterior hardware faster than in almost any other California city. Check deck boards, railings, and stair stringers for rust, loose fasteners, or soft wood. If a fastener has rusted through, replace it with a coated or stainless steel equivalent.

Cost range: most fog-season maintenance is DIY-friendly and costs under $50 in materials. New weatherstripping, silicone caulk, and replacement fasteners are all inexpensive. If you find mold, the economics change: professional mold remediation in an ADU typically runs $1,500 to $4,000 in San Francisco, depending on the size of the affected area and whether drywall needs to be cut out.

Fog season is also a good time to schedule any professional inspections you’ve been putting off. Contractors in San Francisco are generally less slammed in May and early June than they are in the summer, so you’ll get better availability and often better pricing. If you own a prefab ADU from a brand like Abodu, Cover, or Dvele, check your owner’s manual for brand-specific fog-season recommendations; those manufacturers engineer their wall systems differently, but they all benefit from a spring drainage and seal inspection.

Fire-Weather Window: July Through September

From July through September, San Francisco’s climate shifts again. The fog retreats, the winds pick up, and the humidity drops. This is fire-weather season. While San Francisco proper is less exposed than the East Bay or North Bay hills, the city still experiences Red Flag Warnings, and ADU owners should take them seriously, especially if their ADU sits in the wind-exposed western and southern neighborhoods or near any of the city’s parks with significant tree cover. The fire-weather window is also when your cooling system works hardest, and when dry, dusty air exposes any gaps in your ADU’s filtration.

Fire-Weather Maintenance Checklist

1. Clear defensible space around the ADU (2 hours). Remove dead vegetation, leaf litter, and any combustible materials within 5 feet of the structure. Trim back tree branches that overhang the roof. San Francisco’s fire code requires 5 feet of defensible space around all structures; many ADU owners forget this because they think of fire risk as a hills problem, not a city problem.
2. Inspect and clean all exterior vents (30 minutes). Dry winds carry dust and debris into foundation vents, attic vents, and crawlspace openings. Blocked vents reduce airflow and increase moisture buildup. If you have vents with mesh screens, make sure the mesh is 1/8 inch or finer to resist ember intrusion, a requirement in many California jurisdictions.
3. Test the smoke and carbon monoxide detectors (10 minutes). This is not optional. San Francisco requires smoke and CO detectors in all ADUs, and fire-weather season is the right time to replace batteries and test every unit. If your ADU was built before 2014, confirm that your detectors meet current California code, which requires 10-year sealed battery units.
4. Service the mini-split cooling function (1 hour). July through September is when mini-splits in San Francisco ADUs run in cooling mode for the first time since the previous October. Clean the filters, check the condensate drain line, and confirm the unit is producing cold air. If it’s blowing warm or the airflow is weak, call a technician before the next heat wave.
5. Check exterior door and window screens (20 minutes). During fire-weather season, windows stay open more often in San Francisco because there’s no air conditioning in many older units. Screens with holes let in dust, insects, and, during fire season, smoke particulate. Repair or replace damaged screens.

September is also when the “second summer” hits San Francisco. The city’s warmest days often land in late September and early October. If your ADU is a rental unit with tenants, remind them in early September that the cooling system will be running, and ask them to report any issues immediately rather than waiting. A mini-split that’s low on refrigerant will run constantly, drive up the electricity bill, and still not cool the space, and if your tenant doesn’t tell you until the first heat wave, you’ll be dealing with an emergency service call in peak season.

Mini-Split Maintenance: What Title 24 Requires and When to Call

Under California’s Title 24 energy code, new ADUs in San Francisco are required to use high-efficiency heat pump systems for heating and cooling. In practice, that means a mini-split or multi-zone heat pump in almost every new ADU built since 2020. Mini-splits are efficient and reliable, but they require two simple, ongoing maintenance tasks that most owners skip: filter cleaning and condensate drain clearing. Skip both, and you’ll get reduced efficiency, higher bills, and eventually a repair call that could have been avoided.

Filter Cleaning: The 90-Second Task That Saves Hundreds

Mini-split filters are not like central-air filters. You don’t replace them; you wash them. Every 4 to 6 weeks during heavy use (cooling season in San Francisco, which is July through October), remove the filter from the indoor unit, rinse it with water, let it dry completely, and snap it back in. The whole process takes 90 seconds per unit. A dirty filter forces the fan to work harder, reduces airflow, and can cause the evaporator coil to freeze. If left long enough, the coil can ice over completely, and you’ll have water dripping from the indoor unit or no cooling at all.

Condensate Drain: Where the Mold Lives

Every mini-split produces condensate, the water that drips out of the unit when it’s cooling. That water has to go somewhere. In San Francisco ADUs, the condensate line often runs through the wall to the exterior or into a drain line under the unit. If the drain clogs, which happens when dust and mold build up inside the line, the water backs up and leaks out of the indoor unit, typically staining the wall and creating a mold risk. Clean the drain line once a year by pouring a cup of white vinegar through the drain pan, which dissolves mineral buildup and kills mild biological growth. If the drain is already clogged, a technician will need to blow out the line with compressed air or run a small brush through it. That’s a $150 to $300 service call in San Francisco.

When to Call for a Professional Mini-Split Service

• Ice on the coils or refrigerant lines. Visible ice means either low refrigerant or severely restricted airflow. Both require a licensed HVAC technician.
• Warm air in cooling mode after you’ve cleaned the filters. This usually indicates low refrigerant, which means there’s a leak somewhere in the system. Refrigerant handling in California requires certification.
• Water leaking from the indoor unit. This is almost always a clogged condensate drain, but if the drain is clear and it’s still leaking, the unit may be improperly installed or angled wrong.
• Strange noises. Grinding, rattling, or clicking sounds that weren’t there before suggest a failing fan motor, loose mount, or debris in the blower wheel.

Annual service cost range: a professional mini-split cleaning and inspection in San Francisco typically runs $120 to $250 per indoor unit. If you have a multi-zone system with three or four heads, expect to pay $300 to $600 for full service. That’s money well spent, because a well-maintained mini-split lasts 15 to 20 years, while a neglected one can fail at year 7 or 8, and replacement costs $4,000 to $8,000 installed.

Tenant Communication Templates for Every Season

If your San Francisco ADU is a rental, every maintenance task requires coordination with your tenant. San Francisco has strict tenant protection laws, including specific notice requirements before entering a rental unit. Under California law, you must provide at least 24 hours’ written notice before entering a tenant’s unit for non-emergency maintenance. San Francisco’s Rent Ordinance adds additional protections, and if your tenant is in a rent-controlled unit, the rules are even more restrictive. Here are three templates you can adapt for each seasonal maintenance window.

Wet Season Entry Notice (October)

“Hi [Tenant Name], I’ll be doing the annual exterior drainage and weatherproofing inspection on [Date] between [Time] and [Time]. This involves checking the gutters, roof drains, window seals, and exterior caulking. I won’t need access to the interior unless you report any leaks or drafts. Please let me know if [Date] doesn’t work and we’ll find another time. Per California law, this is your 24-hour notice of entry. Thanks!”

Fog Season Entry Notice (May)

“Hi [Tenant Name], I need to do the spring envelope inspection and check the bathroom exhaust fan and window seals on [Date] between [Time] and [Time]. This takes about 45 minutes and I’ll need interior access. I’ll also test the smoke and CO detectors while I’m there. Let me know if you’d prefer a different day that week. This is your 24-hour notice of entry as required by California law. Thanks for your flexibility!”

Fire-Weather Entry Notice (August)

“Hi [Tenant Name], fire-weather season is here, and I need to do a quick safety check on [Date] between [Time] and [Time]. I’ll inspect the exterior vents, test the smoke detectors, and clean the mini-split filter if it needs it. Total time inside is about 30 minutes. Please let me know if there’s a better time that day. This is your 24-hour notice of entry. Appreciate your help keeping the unit safe!”

Key compliance reminders for San Francisco landlords: notice must be in writing (text or email counts as long as the tenant has agreed to electronic communication), you must state the date and a reasonable time window, and you can only enter during normal business hours, which California defines as 8:00 AM to 8:00 PM. If your tenant has a disability or works nights, be flexible on timing. A maintenance entry that violates San Francisco’s notice rules can trigger tenant complaints, and the city takes those seriously.

Common Mistakes to Avoid

• Using an East Coast maintenance calendar. San Francisco’s wet season is the opposite of most of the country. If you follow a guide that says “clean gutters in spring and check the AC in June,” you’ll clean gutters after the rains have already passed and check the AC right before the fog sets in. Build your calendar around San Francisco’s actual climate: wet from October to March, foggy from April to June, dry and warm from July to October.
• Skipping the October dual-prep. Because October is both hot and wet in San Francisco, many owners prep for one and ignore the other. They clean gutters but never test the cooling system, or they service the mini-split but leave the drains clogged. In October, you need to do both.
• Ignoring the flat roof and rooftop deck. National maintenance guides focus on shingles, flashing, and attic ventilation, which are pitched-roof concerns. If your San Francisco ADU has a flat roof or a rooftop deck, the failure points are drains, membrane seams, and deck waterproofing. Inspect those, not shingles you don’t have.
• Treating the tenant as an obstacle. In San Francisco, your tenant is legally entitled to proper notice and a habitable unit. Trying to do maintenance around them without communicating creates conflict and can lead to rent board complaints. Use the templates above, give proper notice, and build a cooperative relationship.
• Deferring mini-split filter cleaning. A dirty filter is the single most common cause of mini-split failure, and the fix takes 90 seconds. Skipping it for a year reduces efficiency by 15 to 25 percent and shortens the life of the unit. Set a recurring calendar reminder.
• Assuming San Francisco doesn’t have real fire risk. The city experiences Red Flag Warnings regularly, and ADUs in the western neighborhoods are exposed to dry winds from July through September. The 5-foot defensible space rule applies citywide, not just in the hills.

When to Call a Professional

Most of the seasonal maintenance in this guide is DIY-friendly, but some situations warrant a professional. Call a licensed contractor if you find active water intrusion that has been happening for more than a week, visible mold covering more than 10 square feet, structural issues like sagging roof decking or cracked foundation elements, electrical problems including flickering lights or warm outlets, or any gas appliance issue. Also call a professional if your mini-split is showing the warning signs described above: ice on the coils, warm air after cleaning filters, or water leaking from the indoor unit. If your ADU is a prefab from a brand like Mighty Buildings or Cover, contact the manufacturer for a list of certified service providers in San Francisco, because their wall and roof systems may have specific inspection requirements you won’t find in a generic guide. Benson ADU San Francisco offers free estimates for ADU repairs and maintenance in San Francisco – call (424) 328-5746 to schedule a walkthrough with a background-checked technician.

Frequently Asked Questions