Why ADU Projects Are Ideal for Radiant
Radiant heating requires exposed foundation or subfloor for tubing installation. In finished homes, this means costly floor demolition. ADU buildouts provide free access to both the foundation level and the ceiling underside being demolished, making PEX tubing installation a minor incremental cost compared to retrofitting later at $15,000 to $25,000 more.
The Three Heating Options for Your ADU
Radiant Floor Heating
Tubing connects to a boiler and runs through the foundation or under the subfloor. Heat rises from the floor, creating even warmth at about five feet with no visible vents or equipment. This system benefits people with asthma or respiratory issues since it avoids forcing air around the room.
Forced Air
Ductwork requires building soffits to conceal ducts, reducing ceiling height and usable space in ADUs. The circulating air can feel drafty and provides less comfort than radiant alternatives.
Mini-Split Heat Pump (Cassettes)
Wall-mounted cassettes provide heating and cooling versatility but present aesthetic compromises. Since most SF ADUs don't require cooling and remain naturally cool at lower levels, this option may be unnecessary.
What It Costs During an ADU Build
| Component | Cost |
|---|---|
| PEX tubing in slab | $4 to $8 per sq ft |
| Manifold and controls | $800 to $1,500 |
| Condensing boiler | $5,000 to $10,000 installed |
| Total for 500 sq ft ADU | $9,000 to $16,000 |
Retrofitting radiant into finished spaces costs $16 to $25 per square foot, plus flooring replacement expenses.
SF ADU Permitting and Radiant
Full ADU projects (basement or garage conversions with foundation work) are ideal for radiant installation since foundation exposure is already part of the process.
Junior ADU projects (carved from existing space, under 500 sq ft) may not involve foundation work. Radiant remains feasible if bathroom additions or floor openings occur, otherwise mini-splits are practical.
ADU Radiant Questions
Do I need a separate boiler for the ADU?
If the main house boiler has sufficient capacity, the system can extend to serve the ADU. Otherwise, a dedicated wall-hung condensing boiler is recommended.
Can I add radiant to just the bathroom?
Yes, and it's common. Even with forced air or mini-splits elsewhere, radiant bathroom floors are popular upgrades at modest cost.
What about radiant cooling?
Hydronic radiant cooling exists but requires careful humidity control to prevent condensation. For Bay Area ADUs, this complexity isn't justified, though radiant heating can pair with mini-split cooling if needed.
