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ADU Cost Guide

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13 min read

Understanding the different factors that influence the cost of ADU projects is crucial for planning and budgeting effectively.

Introduction

Embarking on an Accessory Dwelling Unit (ADU) project is an exciting journey that promises to expand your living space and enhance your property’s value and functionality. Understanding the different factors that influence the cost of ADU projects is crucial for planning and budgeting effectively. At Better Place Remodeling, we’ve been at the forefront of transforming homes across San Diego, guiding homeowners through the maze of ADU costs, from construction types to the slope of your lot. Drawing on our extensive experience in ADU construction and our deep understanding of the San Diego market, we’ve compiled this guide to break down the costs to build an ADU. Let’s dive into how we can help you achieve your dream ADU while aligning with your budget.

Size and Impact on Cost

The size of your ADU significantly influences the overall cost to build an ADU. From our projects, we’ve observed that larger ADUs can benefit from economies of scale, reducing the cost per square foot. However, smaller ADUs might increase the cost per square foot due to fixed expenses that don’t scale down with size.

This table provides an all-in average price range for ADUs of various sizes and how size affects overall construction costs.

ADU Size (sq ft)Average All-In Price*Key Influencing FactorsScenarios
300$120,000 - $175,000Fixed costs such as design permits. Smaller space requires high-quality, multifunctional solutions.A 300 sq ft studio in an urban area might reach the higher end due to premium finishes and compact, efficient appliances.
500$180,000 - $250,000Economies of scale start to apply, reducing per sq ft cost. Choice of finishes and complexity of design play roles.A 500 sq ft 1-bedroom might cost in a suburban setting with standard finishes.
800$250,000 - $350,000Larger footprint allows for more standard construction methods potentially lower costs for additional square footage.An 800 sq ft 2-bedroom could approach $200,000 with higher-end finishes and a custom layout.
1000$330,000 - $465,000Similar to a small home, costs can vary widely based on location, materials, and labor.In high-cost areas, a well-appointed 1000 sq ft ADU might reach or exceed $375,000 due to luxury materials.
1200$415,000 - $580,000Maximum potential for design complexity and high-quality finishes, impacting total cost.A 1200 sq ft 3-bedroom unit with premium amenities like a full kitchen and private outdoor space could be on the high end of the range.

Types and Costs

The type of ADU you choose—whether a garage conversion ADU, detached, attached, or a Junior ADU (JADU)—plays a crucial role in determining the project’s cost. Garage conversion ADUs and JADUs often present more cost-effective solutions. In contrast, detached and attached ADUs might require a more significant investment due to the need for new foundations and utility connections.

Here, we explore how different ADU construction types vary in cost and give a glimpse into budget considerations for each type.

ADU TypeAverage Cost / sq ft*Influencing FactorsScenarios
Detached ADU$390 - $490Foundation work, utility connections, design freedom vs. regulatory restrictions.Building an 800 sq ft detached ADU, including custom design features and high-end finishes.
Attached ADU$380 - $475Integration complexity, access to existing utilities, matching existing home aesthetics.Adding a 600 sq ft ADU attached to the main house, with costs at the higher end for seamless integration.
Garage Conversion$370 - $470Condition of existing structure, complexity of conversion, local codes.Converting a 400 sq ft garage into a living space, costs can vary based on the need for structural upgrades and interior finishes.
Room Addition$405 - $500Extent of structural changes, need for new roofing or foundation extensions.Extending the main home with a 500 sq ft master suite, incorporating luxury bathroom finishes.
JADU (Junior ADU)$475 - $575Use of existing space within the home, minimal structural alterations.Converting part of the primary residence into a 300 sq ft JADU with a basic kitchenette and bathroom.

Sloped Lots

Building on a sloped lot adds complexity and cost to your ADU project. Understanding the impact of terrain on your ADU development is crucial for accurate budgeting.

Below, we detail the additional cost implications based on the slope of your lot, providing insights into the necessary budget adjustments.

Lot SlopeAdditional Cost ImpactChallengesMitigation Strategies
FlatMinimal additional costMinimal challengesNot applicable
Slight Slope+10% to +15%Drainage management, minor grading.Implement simple retaining walls or landscaping to manage water flow.
Moderate Slope+15% to +25%More extensive grading, potential for erosion.Employ engineered retaining walls and significant landscaping efforts to stabilize the slope and manage water effectively.
Steep Slope+25% and aboveAccess for construction, risk of erosion, challenging foundation work.Utilize specialized foundation solutions such as piers or stilts, extensive retaining walls, and professional geological assessments to ensure stability.

Utilities and Interior Finish

The choice of interior finishes and the approach to utilities significantly influence the cost and functionality of your ADU. Opting for basic finishes and efficient utility planning can keep costs down, whereas premium materials and advanced utility systems will increase the investment. Below, we break down how different levels of finishes can impact the cost per square foot of your ADU, from basic to premium.

Finish LevelAverage Cost / sq ftImpact FactorsSustainable Options
Basic$10 - $20Material quality, labor costs. Basic finishes ensure affordability while limiting customization.Use low-VOC paints and recycled materials to enhance eco-friendliness without a significant cost increase.
Standard$20 - $40Choice of materials, complexity of installations. Offers a balance between quality and cost-efficiency.Incorporate energy-efficient appliances, LED lighting, and sustainably sourced wood to improve energy efficiency and lower long-term costs.
Premium$50+High-quality materials, custom design work, brand of appliances. Allows for maximum customization and luxury.Adopt solar water heating, high-efficiency HVAC systems, and smart home technology for optimal energy use and sustainability.

Soft Costs

Soft costs, such as design, permits, and engineering, are essential to your ADU project’s success and can significantly affect the overall cost to build an ADU. This table clearly shows these costs, providing transparency and helping you plan effectively.

ServiceAverage PriceConsiderationsPotential Savings Tips
Design$4,000 - $14,000Design complexity and designer choice affect the price. Higher-end designs and finishes increase costs.Use design-build firms for potential package deals and discounts.
Architectural Plans$2,000 - $6,000Cost varies by ADU scale, and whether plans are custom or pre-designed.Choose standardized plans to lower costs; many firms offer customizable options.
Engineering$1,000 - $5,000Costs depend on structural, electrical, and plumbing needs, project complexity, and local codes.Combine engineering with architectural services for potential discounts.
Permits & Application Fees$3,000 - $11,000Costs reflect local regulations, ADU size, and specific fees like plan checks and utility connections.Check local ADU programs for fee waivers or reductions.
Impact Fees$2,000 - $14,000+Often waived for ADUs under 750 square feet; varies with local ordinances and infrastructure impact.Investigate local ordinances for possible impact fee waivers.
Utility Fees$3,600 - $7,200Costs for connections to sewer, water, and possibly electric and gas; may have fee caps or discounts.Plan utilities early to avoid costly adjustments; consider shared utilities with the main house.
Project Management$5,000 - $20,000Costs influenced by project size and complexity.Use a design-build firm to streamline management and reduce the need for multiple contractors.

ADU Costs Calculator

To help you demystify the costs to build an ADU, use our ADU Costs Calculator. This tool allows you to select the type of ADU, input the estimated square footage, and offer an immediate projection of your project’s costs.

Attached ADU

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Estimated Cost

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*The costs we are calculating here are an average for time and materials. This calculator is meant to provide an estimate to educate you on the ADU building process, please note that this is not a final quote.

Detached ADU

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Estimated Cost

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*The costs we are calculating here are an average for time and materials. This calculator is meant to provide an estimate to educate you on the ADU building process, please note that this is not a final quote.

Garage ADU

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Estimated Cost

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*The costs we are calculating here are an average for time and materials. This calculator is meant to provide an estimate to educate you on the ADU building process, please note that this is not a final quote.

Summary

Now that you’re better informed about the costs involved in ADU construction, you’re in a stronger position to confidently plan your project. Our ADU Cost Guide has laid out the critical components and potential financial considerations, aiming to demystify the budgeting process for homeowners. With this knowledge, you can identify opportunities for efficiency and understand the typical financial commitments required.

For those ready to move forward with their ADU project in San Diego or elsewhere, Better Place Remodeling stands prepared to assist. As a comprehensive, one-stop shop, we eliminate the complexity of managing multiple contractors, making your journey to expanding your living space as smooth as possible. Don’t hesitate to contact us for further assistance or to explore how we can bring your ADU vision to life while aligning with your budget. Let us be your guide and partner in transforming your home with an ADU that meets your needs and exceeds your expectations.

FAQs

The average cost to build an ADU in San Diego can range from $180,000 to $580,000, depending on the size, design, and complexity of the project.

Building a 500 sq ft ADU in San Diego typically costs between $180,000 and $250,000, depending on the chosen finishes and site-specific requirements.

Building a 1000 sq ft ADU in San Diego typically costs between $330,000 and $465,000. This range depends on factors such as the complexity of the design, the type of ADU (detached, attached, or conversion), and the level of finishes chosen.

Key factors include the size of the ADU, the type of construction (detached, attached, garage conversion), the slope of the lot, and the level of finishes and amenities desired.

Yes, converting a garage into an ADU can be more cost-effective, generally ranging from $60,000 to $150,000, depending on the existing structure’s condition and the upgrades needed.

Choosing a full-service provider that manages all aspects of ADU construction can significantly reduce time and coordination efforts, though it might be more costly upfront. In contrast, hiring separate contractors may save on initial costs but can lead to increased project duration and coordination challenges. For efficiency and ease, a garage conversion by a full-service team is often the most streamlined option.

Larger ADUs tend to have a lower cost per square foot due to economies of scale, but overall project costs increase with size. For example, a 1,200 sq ft ADU may cost between $415,000 and $580,000.

Detached ADUs range from $390 to $490 per square foot, attached ADUs from $380 to $475, and garage conversions from $370 to $470 per square foot.

In San Diego County, local regulations significantly affect ADU construction costs due to differences in zoning laws, permit fees, and specific requirements that vary from one municipality to another. For instance, certain areas may require specific types of landscaping, off-street parking, or adherence to historical district guidelines, which can increase overall costs. Additionally, the fees for utilities and the extent of site work required can differ based on local infrastructure and development standards, impacting the total budget for an ADU project.

Yes, sloped lots can increase construction costs by 10% to 25% or more, due to the need for additional grading, excavation, and specialized foundation work.

Budgeting for interior finishes and utilities can vary widely; for basic finishes and utilities, costs might start at $10 per square foot, increasing to $50 or more for premium finishes and advanced utility systems.

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