ADUs in Milton and Alpharetta, GA: Design Variables, Feasibility, Stormwater, Utilities and Fire Flow

A detached guest house, in-law suite or pool house with a bedroom is one of the most requested projects we see in North Fulton. It is also one of the most frequently abandoned, usually after a homeowner has already paid for drawings. The reasons are rarely architectural. They are zoning, stormwater, septic, and fire access. This post walks through the five questions we answer before anyone commits to a design in Milton or Alpharetta. For the statewide picture, start with our Georgia ADU permit guide.

1. Identify your design variables before you commit

The design variables for an ADU are not “how many bedrooms do I want.” They are the handful of numbers in the zoning ordinance that fix the maximum envelope, and they are different in each city. We walk through the variables we check on every ADU in a separate post; here is how they land in these two cities.

Milton

Milton’s Unified Development Code treats a detached accessory dwelling as a guesthouse. The adopted standards, as cited in recent Board of Zoning Appeals cases, are specific: the structure is for relatives, guests or household employees and may not be rented; only one is allowed per lot; a separate kitchen is permitted; heated floor area must be between 650 and 1,500 square feet; it must meet the principal-building setbacks; and it must sit to the rear of the house (Milton UDC Sec. 8.8.13; Milton BZA case V25-01). In AG-1, the district that covers most of Milton’s one-acre-plus lots, the principal setbacks are 60 feet front, 25 feet side, 40 feet side-street and 50 feet rear. On a narrow or shallow lot, those setbacks plus a 1,500-square-foot cap often decide the footprint before an architect draws anything. In Milton’s form-based transect zones around Crabapple and Deerfield, an “accessory dwelling unit” is a separately permitted use with its own standards, so the answer depends on which Milton you are in. Our Milton service page and the Milton subdivision and UDC rules post cover the broader site rules.

Alpharetta

Alpharetta is the harder case, and most online summaries get it wrong. In the conventional residential districts, UDC Section 2.3.3.C lists the uses “customarily accessory to dwellings”: garages and carports, swimming pools and cabanas, sheds and tool rooms, greenhouses, children’s playhouses, pet quarters, home offices and similar items. The list does not include a detached dwelling unit, and it adds two hard limits: no accessory use may be more than 20% of the size of the principal use, and no accessory structure may be closer to the road than the principal structure or in a required front yard (Alpharetta Ordinance, UDC Sec. 2.3.3). A pool house with a bath is clearly accessory. A detached unit with a kitchen and a bedroom is a conversation with Community Development first, and in some cases a conditional-use or variance application, not a building permit application.

The one place Alpharetta explicitly defines a detached accessory dwelling is the Downtown Code, where a carriage house is a permitted building type behind a principal house: one per lot, one bedroom, and a maximum of 700 square feet of heated floor area (Alpharetta UDC Appendix A, Downtown Code). If your lot is in DT-R, DT-LW, DT-MU or DT-C, that is your design envelope. State-level ADU legislation could change this; see our note on Georgia House Bill 1166. Until it does, the 20% rule is the practical cap on anything detached. On a 3,000-square-foot house, that is 600 square feet of accessory structure, total, across every detached building on the lot. See our Alpharetta service page for the rest of the city’s site rules.

Variable	Milton (AG-1 and residential)	Alpharetta (conventional residential)
Detached unit with kitchen allowed?	Yes, as a guesthouse; no rental	Not an enumerated accessory use; carriage house only in Downtown districts
Size cap	650 to 1,500 heated sq ft	20% of principal use; 700 sq ft / 1 bedroom for Downtown carriage house
Count	One per lot	One carriage house per lot (Downtown)
Location	Rear of principal house; principal setbacks apply	Not closer to road than house; not in required front yard; 10 ft side/rear or principal setbacks, whichever is less
Stormwater trigger	5,000 sq ft new impervious, 1 acre, or within 200 ft of state waters	1,000 sq ft new impervious

2. A feasibility plan answers the zoning question before you spend on design

A feasibility study for an ADU is a short, surveyed exhibit that overlays the ordinance on your actual lot. It shows the recorded property lines and easements, the principal-building setbacks, the stream buffers if any, the floodplain line if any, the existing septic tank and drainfield with the required reserve area, the driveway and turning geometry, and the largest footprint that fits inside all of it. On a typical Milton lot it takes a current boundary survey, a Fulton County septic record pull, and a half-day of drafting. It is the cheapest decision document in the project.

It also tells you, in writing, whether you are heading into a variance. Milton’s BZA has heard requests to exceed the 1,500-square-foot cap and to place a guesthouse forward of the house; both are variances from Section 8.8.13 with a public hearing, a staff report and no guarantee (Milton BZA agenda packet). In Alpharetta, anything over the 20% cap or a detached unit outside Downtown is the same kind of discretionary decision. Knowing that before design means you can size the building to the code, or budget the hearing with eyes open, instead of discovering it at plan review.

3. Will the stormwater cost more than the building?

This is the question homeowners never ask and should. Alpharetta applies its post-construction stormwater standards to any new development that creates 1,000 square feet or more of new impervious surface, and for plans submitted since February 2017 the required method is runoff reduction: retain the first 1.0 inch of rainfall on site, with a water-quality alternative only where retention is shown to be infeasible (Alpharetta UDC Sec. 3.3.1). The city’s UDC appendix adds that projects adding between 1,000 and 5,000 square feet of impervious area need soils infiltration testing (infiltration rate, depth to water table, depth to bedrock) before the permit, plus an installation certification and a recorded maintenance covenant (Alpharetta UDC Appendix B).

Run the numbers on a modest project: a 1,200-square-foot guest house, a 450-square-foot driveway extension and a 300-square-foot patio is 1,950 square feet of new impervious surface. One inch of rainfall over that area is about 163 cubic feet of runoff to retain. In a stone infiltration trench at roughly 40% void space, that is around 400 cubic feet of stone, for example a trench 3 feet wide, 3 feet deep and 45 feet long, placed in soil that actually drains and outside the septic drainfield, the stream buffer and the tree save area. On a sandy-loam lot that is a line item. On a Piedmont clay lot with a drainfield in the only flat area, it becomes an engineered system with an underdrain, a maintenance covenant and a real budget. Our stormwater and hydrology engineering team sizes this at the feasibility stage so the number is in the budget, not a surprise, and the ADU grading plan carries it through permit.

Milton follows the Metropolitan North Georgia Water Planning District model: stormwater management is triggered at one acre of disturbance, 5,000 square feet or more of new impervious surface, or any work within 200 feet of state waters (Mack Development, Milton service area). Most single guest houses stay under 5,000 square feet, but the 200-foot stream rule catches many wooded Milton lots, and Milton’s 50-foot undisturbed stream buffer plus 25-foot impervious setback removes a 75-foot band from the buildable area regardless.

4. Utility conveyance to a freestanding building

A detached unit needs water, wastewater, power and usually gas or propane run across the yard, and each one has a rule attached. The septic feasibility and utility due diligence step is where we sort this out.

• Water. In the North Fulton service district, Fulton County Public Works is the water provider for Alpharetta and the served portions of Milton. A guest house can usually be fed from the house’s existing meter, but a long service run to the back of a lot loses pressure, and a 3/4-inch line at 200 feet behaves differently than one at 40 feet. Where a second meter is wanted, the county’s January 2025 schedule lists a 5/8-inch to 3/4-inch new water connection at $3,953 including the capacity fee and meter (Fulton County water connection fees). Large parts of Milton remain on private wells; a well serving two dwellings is a separate conversation with the Fulton County Board of Health.
• Sewer. Where county sewer exists, the question is elevation. If the guest house floor is below the house’s sewer connection or the main, you are buying a grinder pump and a force main, not a gravity lateral. We confirm invert elevations from the survey before the floor is set.
• Septic. Most of Milton and parts of Alpharetta are on septic. The City of Milton provides no water or sewer service; residents are served privately or by Fulton County Water Services, and the city’s own guidance is that a Milton home without a sewer bill is almost certainly on septic (City of Milton, water, sewer and septic information). Georgia sizes residential systems by bedroom count at a design flow of 150 gallons per bedroom per day, assuming two occupants per bedroom (Georgia DPH on-site design flow criteria). A guest house adds bedrooms to the lot. The Fulton County Board of Health Environmental Health Services program, which permits and inspects on-site systems countywide (Fulton County Board of Health), will evaluate whether the existing system and its reserve area can carry the new load or whether a second system is required, and the drainfield and its reserve cannot sit under the new building, the driveway or the infiltration trench.
• Power and gas. A sub-panel fed from the house is the typical approach; a separate service requires utility coordination and a meter location visible from the drive. Trenches for power, gas, water and sewer need separation from each other and from the drainfield, which on a tight lot can dictate where the building goes.

5. Do you have fire flow?

Georgia adopted the 2024 International Fire Code effective January 1, 2026 (Georgia DCA adoption memo). Where a local fire marshal applies Appendix B, a one- or two-family dwelling with a fire-flow calculation area of 3,600 square feet or less requires 1,000 gallons per minute for one hour, and an approved residential sprinkler system allows a 50% reduction (IFC Appendix B, Sec. B105.1). The IFC also requires a fire apparatus access road to reach within 150 feet of all portions of the building, and hydrants within 600 feet of a one- and two-family dwelling where the exception for R-3 occupancies applies. Fulton County will run a hydrant flow test for $128 and return static, residual and available flow at 20 psi (Fulton County fireflow request form).

What this means on the ground: a guest house 250 feet behind a Milton home on a gravel drive may be outside the 150-foot hose-lay distance, on a lot with no hydrant within 600 feet, served by a well. None of those is automatically fatal, but each one is a decision the fire marshal makes, and the answers range from widening the drive and adding a turnaround to a 13D sprinkler system or a water storage tank. Milton Fire-Rescue and the Alpharetta Fire Marshal’s Office review this at plan review, and we would rather have the conversation at feasibility.

Who builds it

Mack Engineering produces the feasibility exhibit, the stormwater design, the utility plan and the site plan that goes to the city. If you want one team through construction, our sister company handles the build: Mack Development builds guest houses and additions in Milton and Mack Development in Alpharetta show current work in both cities. If you already have a builder, we design to their scope and hand off a permit-ready set.

Frequently asked questions

Related reading

• Feasibility Studies & Reports Atlanta GA
• Civil Engineer for ADU & Grading Plans in Georgia
• Accessory Dwelling Units (ADUs) in Georgia
• ADU Permits in Georgia (2026 Guide): Rules by City and County
• Building ADUs in Atlanta GA: Check These Variables First
• Septic System Design Georgia: Soil Reports & Site Limits
• Stormwater Design & Hydrology Studies Atlanta
• Milton GA Subdivision Rules: Splitting Land Under the UDC
• Civil Engineer in Alpharetta, GA: Site Plans & Permits Fast
• Civil Engineer in Milton, GA: Estate Lots & Subdivisions
• Custom Home Builder in Milton, GA (Mack Development)
• Custom Home Builder in Alpharetta, GA (Mack Development)
• Our 5-Step Design-Build Process (Mack Development)

Sources

• City of Milton Unified Development Code, Sec. 8.8.13 Guesthouse and AG-1 dimensional standards
• City of Milton Board of Zoning Appeals, case V25-01 staff report citing Sec. 8.8.13.B
• City of Alpharetta Unified Development Code, Sec. 2.3.3 Accessory uses (ordinance text)
• City of Alpharetta Unified Development Code, Appendix A Downtown Code (carriage house) and Sec. 3.3.1 Stormwater
• City of Alpharetta UDC Appendix B, stormwater design requirements for 1,000 to 5,000 sq ft of new impervious area
• Fulton County Public Works, Water and Sewer Connection Fees, revised January 1, 2025
• Fulton County Public Works, Fireflow Request Form ($128 test fee)
• Georgia Department of Public Health, On-Site Sewage Design Criteria: Peak Sewage Flow (150 gpd per bedroom)
• City of Milton, Water, Sewer and Septic Information
• Fulton County Board of Health, Well and Septic Inspection (On-site Sewage Management Systems)
• Georgia DCA, Notice of adoption of the 2024 ICC code editions effective January 1, 2026
• 2024 International Fire Code, Appendix B Sec. B105.1 (one- and two-family dwelling fire flow)
• Mack Development Group, Milton service area (stormwater triggers and buffers)

This article is general information, not a site-specific engineering opinion. Code requirements and local ordinances change; confirm current requirements with the authority having jurisdiction for your property.