An attached addition to your existing home is usually cheaper than a fully detached ADU because the addition shares a wall, existing electrical service, plumbing lines, and HVAC capacity with the main house. A detached ADU requires new foundation, standalone utility runs, and independent framing, which typically adds 30-50% to the total cost per square foot compared to an attached addition. Detached ADUs typically deliver stronger privacy and resale value, so the decision depends on how the space will be used.