Middleton ADU Guide

Accessory dwelling units shall not exceed 800 square feet or 50% of the primary dwelling's finished square footage, whichever is less.

Investor implication: 800 sqft cap is higher than many Treasure Valley jurisdictions but the 50% primary cap constrains ADU size on smaller primary dwellings. On a 1,200 sqft primary, ADU caps at 600 sqft (not 800). Underwrite ADU rent per actual allowable size, not the 800 sqft headline number.

Detached accessory dwelling units may receive a separate street address for purposes of emergency response coordination and rental management.

Investor implication: Separate street address is a material operational improvement over jurisdictions that require shared address with unit suffix. Simplifies tenant onboarding, utility setup, and emergency service coordination. Verify address assignment process with City Planning before closing.

Parcels located in the extra-territorial zone (Area of Impact) outside Middleton city limits but within the planning jurisdiction are subject to Canyon County overlay regulations. ADU size on ETZ parcels is capped at 500 square feet.

Investor implication: ETZ jurisdiction is the dominant ADU diligence risk in Middleton. A parcel that appears to be 'in Middleton' on a map may be governed by Canyon County overlay if it's outside city limits. 500 sqft ETZ cap vs. 800 sqft in-city cap is a 37.5% reduction in rentable area. Verify jurisdiction with Canyon County Assessor parcel lookup before any ADU underwriting on Area of Impact land.

Cities with populations over 10,000 must allow at least one accessory dwelling unit per lot. Owner-occupancy requirements are prohibited. HOAs may not add new ADU restrictions without written consent of all members; existing HOA bans remain enforceable.

Investor implication: SB 1354 preempts local owner-occupancy requirements but does not override existing HOA covenants. On any Middleton parcel with an HOA, verify CC&Rs for ADU restrictions before acquisition. Post-July 2026, HOAs cannot add NEW ADU bans, but pre-existing bans remain enforceable.