ADU rules in Mason City

Mason City conformed its zoning code to the 2025 state ADU law with Ord. 25-12 (final passage 11-4-2025), which added §12-16-24. In the Z1, Z2, Z3, Z7-5 and Z7-6 districts, on any lot where a freestanding house is the principal function, one ADU 'shall be allowed', either freestanding as a separate accessory building or inside a detached garage. These are the districts where the form and function table permits a freestanding house, which matches the state rule (as amended by SF 2369 effective 7-1-2026) that an ADU must be allowed wherever a single-family residence is an allowed principal use. A detached ADU must meet the house's development standards and the ADU setback table (Table 3).

Section 12-16-24(A)(3)(c) allows an ADU 'located within the principal structure, provided that there is direct access between the accessory dwelling unit and the primary dwelling unit', and the §12-2-2 definition covers a unit 'a part of the same building as the primary freestanding house'. Basement, attic and addition units therefore qualify in the Z1, Z2, Z3, Z7-5 and Z7-6 districts. The direct-access condition is a local design rule the statute does not contain; Iowa Code §364.3(23)(e) defines an ADU as one 'either attached to or detached from' the house, and paragraph (b)(1) bars placement or appearance rules stricter than those for the house, so enforcement of that condition against an otherwise code-compliant attached unit is open to challenge.

Section 12-16-24(A)(2) requires at least 400 gross sq ft and caps the ADU at 1,000 gross sq ft 'or fifty percent (50%) of the gross ground floor area of the primary dwelling (excluding garages, porches, and other unheated spaces), whichever is greater'. The state floor measures 50 percent of the residence's 'size', defined since 7-1-2026 (SF 2369) as the primary dwelling's gross living area excluding garages, decks and unheated porches, and the ADU's own size now excludes unfinished basements. For a multi-story house over 2,000 sq ft of living area the local ground-floor measure yields a smaller cap than the statute, and to that extent the local text is void under §364.3(23)(d). Height follows the house: the Z1 and Z2 tables allow 2 stories and the Z3 table 3 stories for an ADU, equal to the principal building, and detached units use the Table 3 ADU setbacks (in Z3, 10 ft front, 5 ft rear).

Neither §12-16-24 nor the §12-2-2 definition requires the owner to live on the property, and Ord. 25-12 added no residency rule. Iowa Code §364.3(23)(b)(4) separately bars a city from 'requiring the property owner to be a resident' or imposing familial, employment, income or age occupancy limits on an ADU, so no owner-occupancy requirement can apply at application or afterward.

Section 12-16-24 sets no ADU parking rule. The general Table 4 in §12-8-5 requires residential parking 'per dwelling' (2 in Z1, 1.5 in Z2, 1 in Z3), which read literally would add spaces for an ADU, but Iowa Code §364.3(23)(b)(3) bars requiring 'additional parking beyond that required for a single family residence or payment of a fee in lieu', so that per-dwelling count is void as applied to an ADU. The house's own requirement (for example 1 space in Z3) still applies.

The form and function table lists 'Accessory dwelling' as P (permitted) in Z1, Z2 and Z3, and Table 1A lists it as P in the Z7-5 and Z7-6 columns; under §12-8-2(A) a P use is 'Permitted subject to administrative site plan compliance approval', with no commission or board hearing. A building permit under the Title 10 building codes follows. This matches Iowa Code §364.3(23)(c), which requires approval 'without discretionary review or hearing' on the single-family timeline, with written reasons for any denial.

Mason City's Housing Code requires a rental dwelling permit for every rented dwelling unit (§10-2F-10), issued after inspection and valid up to five years; this general program applies to an ADU like any other rental and is lawful under §364.3(23)(b)(2). The zoning code treats short stays as 'residence lodging' (permitted in Z1 to Z4) and 'bed and breakfast', definitions that require operation by the property owner or an on-site resident manager, and Table 4 assigns lodging 1 parking space per room. Iowa Code §414.1(1)(e) bars any city regulation, conditional use requirement, licence or permit fee aimed at short-term rental properties (30 days or less) and classifies them as residential land use, so those owner-operator and per-room parking conditions cannot be enforced against a short-term rented ADU; only same-as-other-property safety, nuisance and emergency-contact rules survive. No separate short-term rental licence was found in the code.