Thinking about a backyard cottage, a home office, or a place for family right on your 78745 property? You are not alone. With updated city rules and a wide range of lot sizes in south Austin, the right accessory dwelling unit can add space and long-term value. In this guide, you will learn how to design an ADU that fits 78745 lots, meets Austin’s rules, and supports your lifestyle and budget. Let’s dive in.

What fits 78745 lots

Most single-family lots in 78745 were built out in the 1950s to 1970s and commonly measure about 5,700 to 10,000 plus square feet. That size usually supports a modest backyard unit, a garage conversion, or an attached suite. Many properties have mature trees, so plan early for root zones and placement to avoid delays. Older drainage patterns and utility locations can also shape where an ADU sits and what it costs to connect.

Know the rules first

HOME changed the playbook

Austin’s Home Options for Mobility and Equity, or HOME, broadened what you can build on many single-family lots, including allowing up to three units in certain cases. Review whether your design is treated under the HOME framework or under the existing single-family standards before you draft plans. You can start with the city’s overview of the HOME amendments.

Size and separation basics

As a working rule, plan for the smaller of 1,100 square feet or 15 percent of your lot area for the ADU’s size. If you add a second story, many examples show a practical limit around 550 square feet of conditioned space on that upper level. You also need separation from the main house, which is commonly about 10 feet for a detached unit. See common sizing guidance used locally in this Austin ADU overview.

Parking and access

Austin reduces or waives extra parking for ADUs near certain transit corridors. If your lot is within a quarter mile of an Imagine Austin activity corridor served by transit, you may not need to add off-street parking. Otherwise, one off-street space is often required, so confirm your status with the city. For background on the parking changes, see this overview of Austin’s ADU parking policy.

Short-term rentals

ADUs are typically eligible for long-term rental, while short-term rental rules are separate and more restrictive. If you plan to host short stays, review the city’s short-term rental licensing guidance before you design.

Permits and timeline

Plan on design, submittal, review, corrections, permit issuance, then inspections. Austin’s Residential Plan Review publishes standard timelines and offers preliminary plan review meetings that can surface issues early. Start with the city’s Residential Plan Review page to understand forms, timing, and fees.

Parcel checks that matter

• Deed restrictions or HOA rules can limit ADUs even when the city allows them.
• Heritage trees have strong protections, which can change your layout.
• Special overlays, including Wildland-Urban Interface areas, may add standards. The HOME amendments page notes rollout details and special areas to verify.

ADU types that work in 78745

Detached backyard cottage

A single-story studio or one-bedroom in the 400 to 900 square foot range is a natural fit on deeper lots. It offers privacy for guests or tenants and keeps construction away from the main house. Keep forms simple, use low eaves, and orient entries for privacy. For design inspiration and practical build tips, see this Austin feature in Fine Homebuilding.

Garage conversion or over-garage unit

Converting a garage or building above it can save foundation and site-work costs. Plan for structural checks, sound separation, and safe stairs if you build up. If you remove required parking, confirm whether your address qualifies for a parking reduction. The city’s Residential Plan Review page outlines what to submit and how reviews work.

Attached in-law suite

An addition with a separate entrance can be cost-effective and meet multigenerational needs. Depending on how you permit it, different standards may apply, including Subchapter F for single-family forms. You can review code pathways via the city’s Land Development Code resources.

Small multi-unit options

On larger lots, some owners are exploring two or three units under the HOME rules. These projects require careful planning for utilities, parking, and site circulation, and they follow different standards than a simple backyard cottage. Learn what is possible on the HOME amendments page.

Accessibility and aging in place

For family care or your future needs, prioritize a single level, zero-step entries, wider doors, and accessible bathrooms. Simple early design choices can add safety and comfort with modest cost.

Size and layout quick math

• If your lot is 6,000 square feet, 15 percent gives you 900 square feet as a working cap.
• If your lot is 7,500 square feet, 15 percent is 1,125 square feet, so the 1,100 square foot cap would apply.
• For two-story ADUs, keep upper-floor conditioned area modest. Many examples use about 550 square feet on the second level, then put living and sleeping on the main floor.

Keep building coverage, impervious cover, tree root zones, and the typical 10-foot separation in mind as you place the unit.

Costs, financing, and timing

Costs vary widely based on size, finishes, site work, and utilities. Recent Austin ranges show small detached ADUs around 400 to 600 square feet often running about $120,000 to $200,000, mid-sized units at $160,000 to $260,000, and larger or higher-finish units from $230,000 up. Conversions can be lower when the structure and utilities cooperate. For a market snapshot, see this Austin ADU cost summary.

Common financing paths include cash, a HELOC, a cash-out refinance, and construction or renovation loans. Lenders may consider anticipated rental income, but underwriting varies by program.

Plan for about 9 to 18 months from design to move-in, depending on complexity and whether you use prefab elements. City review cycles, corrections, site work, and inspections all add time. The city’s Residential Plan Review outlines review timelines. Street Impact Fees often do not apply to ADUs on lots with an existing house, since ADUs typically do not trigger the trip threshold, but review the city’s Street Impact Fee information for your situation.

Key cost drivers to watch include utility run length and service upgrades, tree mitigation, grading or retaining walls, and any new driveway or screening requirements. A contingency of 10 to 20 percent is a smart buffer, as noted in this Austin cost breakdown.

78745 ADU property checklist

• Verify whether HOME rules or single-family standards apply to your lot using the city’s HOME amendments overview.
• Calculate size using the 1,100 square foot or 15 percent rule to set your design envelope. See local sizing guidance in this Austin ADU overview.
• Map trees and likely root protection zones before choosing a footprint.
• Confirm water, sewer, and electric capacity early. A longer run or panel upgrade can add cost.
• Check parking needs or eligibility for a transit-area reduction. This ADU parking summary explains the policy context.
• Review deed restrictions or HOA rules for any private limits.
• Budget permitting, design, utility allowances, and a contingency. An Austin-focused cost review is here: How much does an ADU cost.

Ready to turn your 78745 lot into a flexible home base or income-producing asset? If you want research-backed guidance on rules, design fit, and value, let’s map a plan that fits your goals and timeline. Connect with Cody Hobza to get started.

FAQs

Can I build an ADU on a typical 78745 lot?

• Often yes, since many lots measure roughly 5,700 to 10,000 plus square feet, but you must confirm parcel rules, setbacks, tree protections, and utility capacity with the city.

How big can my ADU be in Austin?

• Use the smaller of 1,100 square feet or 15 percent of your lot as an upper limit, and note that second floors and site coverage rules can reduce practical size.

Do I need extra parking for an ADU in 78745?

• Maybe not, because lots within a quarter mile of a transit-served activity corridor can qualify for reduced or zero additional spaces, otherwise one off-street space is often required.

How long does ADU permitting take in Austin?

• Plan for several weeks to months of plan review and corrections plus inspections, with standard review cycle times published by the city for residential projects.

What does an ADU cost in Austin right now?

• Recent ranges show small detached units around $120,000 to $200,000, mid-sized $160,000 to $260,000, and larger or higher-finish builds from about $230,000 up, with conversions often costing less.