Detached vs. Attached ADU: Which Is Better?
Neither is universally better. Detached ADUs offer privacy and simpler fire separation but cost more with their own foundation and utility runs. Attached ADUs and conversions are cheaper to build but share walls, noise, and trickier fire-rated separation with the main house.
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Cost comparison
Detached ADUs typically cost more per square foot: new foundation, new roof, longer utility trenches, and standalone structural and energy systems. Attached ADUs share foundations, walls, and roof structure with the main house. Conversions of existing garages or basements are usually the cheapest path since the shell already exists. The spread between cheapest and most expensive can be very wide.
Privacy, noise, and rental appeal
Detached units rent better in most markets because tenants value separation: no shared walls, independent entrances, and a sense of their own home. Attached ADUs work well for family members but generate more noise and privacy friction as rentals. If rental income is the goal, the detached premium often pays for itself.
Code and utility differences
Detached ADUs face full new-construction requirements for structure, energy, and sometimes sprinklers and solar. Attached units must solve fire-rated separation between the units and shared-wall sound and energy details. Utility metering choices differ too: detached units more often get separate meters, while attached units commonly share services.
The bottom line
Match the type to the goal. Maximum rental income favors detached; lowest construction cost favors attached or conversion.