What it costs to build an ADU in Mamaroneck, NY

One-Family Residence District - Floor area follows the lot-size schedule, 0.370 at 10,000 sq ft, running from 0.530 on a lot under 5,000 sq ft down to 0.270 at 20,000 sq ft or more, interpolated between the listed sizes.

• The minimum habitable floor area is 1,400 sq ft for a one-story dwelling and 1,600 sq ft for a two-story dwelling.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.
• The owner of a corner lot designates once, in writing, whether both street lines are front lines or one is the front and the other a side line.
• With site plan approval, a planned residential development on at least 10 acres may have attached dwellings of up to eight units per building and the lot, yard and coverage figures waived.

One-Family Residence District - Floor area follows the lot-size schedule, 0.320 at 15,000 sq ft, running from 0.530 on a lot under 5,000 sq ft down to 0.270 at 20,000 sq ft or more, interpolated between the listed sizes.

• The minimum habitable floor area is 1,650 sq ft for a one-story dwelling and 1,800 sq ft for a two-story dwelling.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.
• With site plan approval, a planned residential development on at least 10 acres may have attached dwellings of up to eight units per building and the lot, yard and coverage figures waived.

One-Family Residence District - Floor area follows the lot-size schedule, 0.270 on a lot of 20,000 sq ft or more; the ratio runs from 0.530 on a lot under 5,000 sq ft down to 0.270 and is interpolated between the listed sizes.

• The minimum habitable floor area is 1,650 sq ft for a one-story dwelling and 1,800 sq ft for a two-story dwelling.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.
• With site plan approval, a planned residential development on at least 10 acres may have attached dwellings of up to eight units per building and the lot, yard and coverage figures waived.

One-Family Residence District - Floor area follows the lot-size schedule, 0.530 at 5,000 sq ft, running from 0.530 on a lot under 5,000 sq ft down to 0.270 at 20,000 sq ft or more, interpolated between the listed sizes.

• The minimum habitable floor area is 1,100 sq ft for a one-story dwelling and 1,400 sq ft for a two-story dwelling.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.
• With site plan approval, a planned residential development on at least 10 acres may have attached dwellings of up to eight units per building and the lot, yard and coverage figures waived.

One-Family Residence District - Floor area follows the lot-size schedule, 0.480 at 6,000 sq ft, running from 0.530 on a lot under 5,000 sq ft down to 0.270 at 20,000 sq ft or more, interpolated between the listed sizes.

• The minimum habitable floor area is 1,300 sq ft for a one-story dwelling and 1,500 sq ft for a two-story dwelling.
• Some R-6 lots are mapped R-6/O-1, a combined designation with the O-1 Office District, which also allows the uses of the one-family district the Zoning Map shows as its alternate.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.

One-Family Residence District - Floor area follows the lot-size schedule, between 0.430 at 7,000 sq ft and 0.410 at 8,000 sq ft, running from 0.530 on a lot under 5,000 sq ft down to 0.270 at 20,000 sq ft or more, interpolated between the listed sizes.

• The minimum habitable floor area is 1,400 sq ft for a one-story dwelling and 1,600 sq ft for a two-story dwelling.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.
• With site plan approval, a planned residential development on at least 10 acres may have attached dwellings of up to eight units per building and the lot, yard and coverage figures waived.

Parking District - Every yard is set by the Planning Board at a minimum of 3 ft and a maximum of 20 ft.

• The use is limited to off-street parking accessory to a use on adjoining property, in a residential district for a residential P District and in a nonresidential district for a nonresidential one.
• Neither may store vehicles for sale or hire or service them.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.

One- and Two-Family Residence District - The lot minimum here is stated only per dwelling unit, 3,750 sq ft for each unit, and it applies to one-family and two-family dwellings alike.

• Each dwelling unit needs at least 900 sq ft of habitable floor area.
• Floor area is capped by a lot-size schedule, from 0.530 on a lot under 5,000 sq ft down to 0.270 at 20,000 sq ft or more, interpolated between the listed sizes.
• No side yards are required between attached dwellings.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.

One- to Four-Family Residence District - The lot minimum here is stated only per dwelling unit, 2,500 sq ft for each unit, and it applies to every dwelling of one to four families.

• Each dwelling unit needs at least 900 sq ft of habitable floor area.
• No side yards are required between attached dwellings.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.

One- to Four-Family Residence District - These lots carry the combined designation R-4F/P. For parking use the P row governs: a 5,000 sq ft lot, 50 ft of width, 10% building coverage, 50% for buildings and impervious surfaces, 35 ft, and yards of at least 3 ft and at most 20 ft.

• Parking use is limited to off-street parking accessory to a permitted use on adjoining residential property.
• For a dwelling the lot must hold 2,500 sq ft for each dwelling unit, and each unit needs at least 900 sq ft of habitable floor area.
• No side yards are required between attached dwellings.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.
• Where buildings on the same side of the block within 150 ft show a pronounced uniform front yard, a new building must follow it as nearly as practicable, but need not set back more than 50 ft.
• An accessory building must meet the front and side yards of the lot and stand at least 6 ft from the rear lot line.

Multiple Residence District - Each dwelling unit needs 400 sq ft of usable open space, which excludes parking, though rooftop and atrium space may count for up to 10% of it.

• No building may be longer than 160 ft.
• The district also permits the uses of a one-family residence district.
• The least dimension of a court must equal the height of the wall above the sills of the lowest windows it serves, at least 20 ft for an outer court and 40 ft for an inner court.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.
• No side yards are required between attached dwellings.
• Land in the bed of a street, and land below the high-water mark of Long Island Sound or an adjacent pond or stream, does not count toward lot area, yards, coverage or floor area.

Multiple Residence District - These lots carry the combined designation RM-1/P. For parking use the P row governs: a 5,000 sq ft lot, 50 ft of width, 10% building coverage, 50% for buildings and impervious surfaces, 35 ft, and yards of at least 3 ft and at most 20 ft.

• Parking use is limited to off-street parking accessory to a permitted use on adjoining residential property.
• Each dwelling unit needs 400 sq ft of usable open space, which excludes parking, though rooftop and atrium space may count for up to 10% of it.
• No building may be longer than 160 ft.
• The district also permits the uses of a one-family residence district.
• No side yards are required between attached dwellings.
• Buildings on the same lot must stand at least as far apart, where they are nearest, as their average height there.

Multiple Residence District - With site plan approval the Planning Board may raise the permitted number of dwelling units by 20% for below-market housing, and by a further 10% where at least two thirds of the units are studios or one-bedroom units, on a finding of no seriously adverse school impact.

• Each dwelling unit needs 300 sq ft of usable open space, which excludes parking, though rooftop and atrium space may count for up to 10% of it.
• The schedule allows 3 1/2 stories on wide streets.
• No building may be longer than 185 ft.
• On Mamaroneck Avenue and Boston Post Road the Planning Board may approve a half floor covering no more than 50% of the footprint, set back from at least two sides 1 ft for every 1 ft of height.
• The district also permits the uses of a one-family residence district.
• No side yards are required between attached dwellings.

Multiple Residence District - With site plan approval the Planning Board may raise the permitted number of dwelling units by 20% for below-market housing, and by a further 10% where at least two thirds of the units are studios or one-bedroom units, on a finding of no seriously adverse school impact.

• Some RM-3 lots are mapped RM-3/O-1/C-1, a combined designation with the O-1 Office and C-1 General Commercial Districts.
• Each dwelling unit needs 150 sq ft of usable open space, which excludes parking, though rooftop and atrium space may count for up to 10% of it.
• The schedule allows 4 1/2 stories on wide streets.
• No building may be longer than 185 ft.
• On Mamaroneck Avenue and Boston Post Road the Planning Board may approve a half floor covering no more than 50% of the footprint, set back from at least two sides 1 ft for every 1 ft of height.
• The district also permits the uses of a one-family residence district.

Multiple Residence/Senior Citizen District - Senior citizen housing here is limited to projects of local nonprofit sponsors with state or federal assistance, occupied by persons 62 or over or handicapped persons and the household members the code lists.

• A lot must be at least 20,000 sq ft, with at least 300 sq ft of lot per dwelling unit and at least 415 sq ft of habitable floor area.
• Two-bedroom units may not normally exceed 15% of the units.
• The height limit is 75 ft, at 8 stories, or 4 1/2 on wide streets.
• Walks, handrails, fixtures and elevators must meet the design standards for the elderly and handicapped.
• With site plan approval the Planning Board may raise the unit count by 20% for below-market housing, and by a further 10% where at least two thirds of the units are studios or one-bedroom units.
• The Planning Board sets the usable open space during site plan approval.

General Commercial District - Residence uses need a Planning Board special permit. As a principal use they need a site under 40,000 sq ft, a floor area ratio of 0.60 for four or fewer units or 0.80 for five or more, 100 ft of depth, 450 sq ft per unit, 3 stories and 35 ft, and 30% coverage.

• Those residence yards are 20 ft front, 10 ft on the lesser side, 20 ft for both sides and 25 ft rear, with a 5 ft landscaped parking setback and 300 sq ft of open space per unit.
• Coverage may reach 35% where every unit is fair and affordable, and a development of five or more units must include fair and affordable units.
• On Boston Post Road a residence may only be accessory to a permitted use: one story of residential use, 25 ft and 30% coverage.
• A residence as a principal use is also barred on the southwestern side of Old White Plains Road northwest of Center Avenue and on Mamaroneck Avenue north of Nostrand Avenue.
• Within 45 ft of a residence district boundary the side or rear yard must be 45 ft, though not next to a P District; elsewhere a yard that is provided must be at least 5 ft.
• That 45 ft yard may be reduced to 10 ft where the wall facing the residence district has no openings and the reduced yard is not used for parking, loading or any other activity.

Central Commercial District - Height is capped at 4 stories and 45 ft, or 6 stories and 60 ft for qualifying below-market-rate housing, and the 2.0 floor area ratio can be raised further for below-market-rate housing under Article XV.

• Residence uses need a Planning Board special permit and a site under 40,000 sq ft with at least five dwelling units in the building, and are held to 40 ft.
• No ground-floor space fronting Mamaroneck Avenue may be residential.
• Within 45 ft of a residence district boundary the side or rear yard must be 45 ft, though not next to a P District; elsewhere a yard that is provided must be at least 5 ft.
• That 45 ft yard may be reduced to 10 ft where the wall facing the residence district has no openings and the reduced yard is not used for parking, loading or any other activity.
• A development of five or more units must include fair and affordable units.
• On a corner lot the Planning Board sets the street setbacks, and a front yard of at least 10 ft applies along Fenimore Road.

Manufacturing District - No front, side or rear yard is required as a baseline; on a corner lot the Planning Board sets setbacks case by case.

• A front yard of at least 10 ft applies along Fenimore Road.
• New residential construction and additions must keep 15% of the lot as usable open space.

General Marine - Commercial District - Within 45 ft of a residence district boundary the side or rear yard must be 45 ft, though not next to a P District. In the Marine Zone the rear yard may be reduced to 0 ft for structures used only for hauling and launching boats.

• That 45 ft yard may be reduced to 10 ft where the facing wall has no openings and the yard is not used for parking, loading or other activity.
• Elsewhere a yard that is provided must be at least 5 ft, and a side yard used for access to parking must be at least 20 ft.
• A clubhouse, pool and the sale of food and beverages to boaters are allowed only as accessory to a principal use on a lot of 5 acres or more.
• Those accessory uses and their parking may take no more than 20% of the lot, a clubhouse no more than 5%, and a restaurant no more than 2,500 sq ft.
• They must be set back 50 ft from adjoining residential property or a public street, 150 ft from a residential cluster development, and may be no taller than 2 1/2 stories or 35 ft.
• New residential construction and additions must keep 15% of the lot as usable open space.

Central Marine - Commercial District - For a tiered development, maximum coverage is 80% below mean curb level and 50% above it.

• No non-water-dependent use is permitted unless a floor area ratio of at least 1.0 is used for water-dependent uses, and none is permitted below mean curb level.
• No water-dependent use existing when the Marine Zone was established may be converted.
• Within 45 ft of a residence district boundary the side or rear yard must be 45 ft, though not next to a P District; elsewhere a yard that is provided must be at least 5 ft.
• That 45 ft yard may be reduced to 10 ft where the facing wall has no openings and the yard is not used for parking, loading or other activity.
• In the Marine Zone the rear yard may be reduced to 0 ft for structures used only for hauling and launching boats.
• The 3 stories and 40 ft are both measured from mean curb level.

Marine Recreation District - At least 25% of the lot area must remain open space, and a separate Long Island Sound setback applies under § 146-7 (Coastal Management).

• That open space is grass, landscaped areas open to the sky and beach within the property line, but not land under water, courts, paved recreation or parking areas, courtyards or decks.
• A side yard used for access to parking must be at least 20 ft.
• New residential construction and additions must keep 15% of the lot as usable open space.

Public District - In PB, all Village property is put to such public uses as the Village Board may determine (§ 342-34).

• The code sets no dimensional standard for the district.
• Dedicated parkland may be used only for public recreation.
• Village uses of Village-owned property are not subject to the chapter limitations.

Office District - The uses of the one-family residence district that the Zoning Map shows as the alternate to O-1 are also permitted, and on the map the district appears only in such combinations, such as R-6/O-1 and RM-3/O-1/C-1.

• Offices, research and development laboratories, hospitals and nursing homes, and transformer stations each need a special permit.
• Parking and loading are prohibited in front yards and within 10 ft of any street or lot line, and open parking and loading areas must be screened as the Planning Board requires.