What 62 Homeowners Built at 800 Square Feet — And What It Cost

Eight hundred square feet is where the renovation conversation changes register. Every project type below this size involves some degree of optimization: making the most of a compact footprint, designing efficiently, choosing the right trade-offs. At 800 square feet, that pressure largely disappears. A detached ADU can deliver two bedrooms and two baths, a rooftop deck, and finishes that rival a boutique apartment building. A primary suite can include a dressing room, a spa bath with fixtures that would look appropriate in a five-star hotel, and private outdoor access to a dedicated deck. A kitchen and family room expansion can include a professional appliance suite alongside a seating area large enough to host a dinner party.

Based on 62 closed Realm projects, the median cost is $338,044 and the average is $399,965. Both figures represent a meaningful step up from smaller size brackets, reflecting the premium character that defines most projects at this scale. The moderate spread between median and average reflects reasonable consistency in project type and complexity across the dataset, with a handful of high-complexity builds at the upper end pulling the average above the median.

Key Takeaways

• At 800 square feet, projects enter premium territory. Rooftop decks, 2BR/2BA configurations, spa-level primary suites, and professional appliance suites all appear in the project data at this size for the first time.
• The median project cost is $338,044, with an average of $399,965. Both figures are substantially above smaller size brackets, reflecting the premium character of most projects at this scale.
• 800 square feet is the practical ceiling for a garage conversion that stays within the original footprint. Above this size, extensions beyond the existing structure almost always become necessary, changing both the cost and the permitting picture.
• A 2BR/2BA configuration becomes achievable in a detached ADU at this size, producing a unit that commands rents at the top of the ADU category and attracts tenants who plan to stay.
• Primary suite additions at 800 square feet reach spa-level, with room for a dressing room, separate fixtures, and private outdoor access that makes the addition function as a resort-quality private retreat.
• 62 projects provide a solid foundation for the cost benchmarks here, though the smaller dataset compared to peak brackets means individual project variation can be significant at this scale.

Where Renovation Ends and Premium Living Begins

Eight hundred square feet is approximately 20 by 40, or a more proportional 25 by 32. At this size, the spatial experience stops being about how well a space is used and becomes about how well it has been designed. A two-bedroom, two-bath apartment at 800 square feet has room for both bathrooms to feel private and genuinely well-appointed rather than just code-compliant. A kitchen and family room addition has room for a professional range alongside a seating area for twelve people. A primary suite has room for a dressing room that functions as an actual room rather than a large closet.

The most useful reference point for most people is a well-designed two-bedroom apartment in a newer luxury building, the kind where the quality of every space reflects the overall investment rather than a budget that ran out before the finishes were chosen. That's what 800 square feet delivers when it's been designed at the level these projects typically demand.

Experience the Scale Before You Set the Budget

Mark out a 25 by 32 rectangle somewhere you can move around inside it before any planning conversations begin. The value of this exercise at 800 square feet is different from smaller sizes: it's not about confirming that things fit. It's about understanding what generous means at this scale. Stand at one end and look toward the other. Picture two bedrooms at opposite ends, each with its own bathroom, a kitchen with a full professional range and an island with seating, and a living area that could accommodate a sectional without touching the dining space. At 800 square feet, that layout has room to spare. The exercise recalibrates the planning instinct from "how do we fit this in?" to "how do we make this exceptional?"

What Six Project Types at This Size Tell Us

With 62 projects across six distinct categories, the 800 sq ft bracket has a narrower project mix than smaller size brackets. That narrowness is itself meaningful: homeowners building at this scale and cost level tend to arrive with a clearer and more specific goal than at more entry-level footprints. The distribution skews heavily toward ADU-type projects, which reflects the straightforward reality that at 800 square feet, an ADU is a premium living product, and the investment required to build one is most clearly justified by the income and value it generates. Learn how Realm helps homeowners clarify their goal and build an accurate scope before any contractor conversations begin.

Six Project Types, One Clear Theme

Six project types appear in the data at this size. The most frequently built are detached ADUs and garage conversion ADUs, followed by kitchen expansions, primary suite additions, bedroom additions, and basement ADUs. The concentration of ADU-type projects at this size is higher than at any previous bracket, driven by the same logic that has pushed ADU frequency upward throughout the series: as the footprint grows, the case for a complete, premium living unit becomes most clearly justified by the economics of what that unit produces.

Each Project Up Close: Specs, Costs, and What to Get Right

Detached ADU: The Premium Configuration That Justifies the Investment

A detached ADU at 800 square feet is the premium version of what the dataset has been building toward across every previous size bracket. A two-bedroom, one-bath layout at this size is spacious in a way that tenants notice and stay for. A two-bedroom, two-bath starter configuration becomes achievable for the first time in the dataset, giving each bedroom its own bathroom, which commands meaningfully higher rents across Realm's markets and reduces vacancy to the kind of levels that make the investment genuinely perform over time. A rooftop deck, where structural and lot conditions allow, appears as a genuine feature in the project data at this size rather than an exceptional outlier.

Premium finishes are typical at this size because homeowners building at 800 square feet are almost always making a deliberate, long-horizon investment in the property. The rental income potential of a 2BR/2BA detached ADU with quality finishes in Los Angeles, the Bay Area, San Diego, or Seattle is substantial enough to produce strong long-term returns even at the cost premium this size and finish level require.

The scope variable with the most budget leverage: Full MEP runs, a substantial new foundation, site work, and premium finish coordination. A detailed, accurate scope before contractor conversations begin is more important here than at any smaller size bracket, because the cost of a mid-project change at this investment level is proportionally significant. Realm's advisors help homeowners build that scope before any contractor conversations begin.

Garage Conversion ADU: The Practical Ceiling of the Conversion Model

Eight hundred square feet is the practical ceiling for a garage conversion that remains meaningfully within the original garage footprint, and the data reflects it directly. At this size, the project delivers a comfortable two-bedroom, one-bath unit or a spacious single-bedroom layout with a proper island kitchen and genuine living area separation. Some projects at this size do extend slightly beyond the original footprint to accommodate the full square footage, which is why this is described as the upper boundary of the true conversion category.

What changes above 800 square feet is significant: conversions that extend meaningfully beyond the original garage structure require new foundation sections, exterior framing, and roofing tie-ins that begin to look more like a room addition than a conversion. The cost efficiency that defines garage conversions as a project type starts to erode above this threshold, which makes 800 square feet a meaningful planning benchmark for homeowners who want the efficiency of a conversion while maximizing unit size.

The question that determines whether this project type retains its advantage: How much of the 800 sq ft scope fits within the existing garage envelope versus how much requires extending beyond it. That answer shapes the cost picture significantly. A Realm advisor can help you assess what's achievable within your specific garage structure before you commit to a target square footage.

The parking reality that needs an honest evaluation: At 800 square feet, you're converting a full two-car garage. Losing covered parking for two vehicles has different implications in different neighborhoods in Realm's markets. Think through your specific situation before the permits are filed.

Kitchen Expansion: When a Great Room Reaches Its Highest Expression

A kitchen expansion at 800 square feet is the upper end of the great room transformation that Realm's data has documented across smaller size brackets, and at this footprint it's delivered at its most complete and most premium. A large kitchen combined with a family room addition at this scale accommodates a high-end professional appliance suite, an island with seating for six or more, a dining area that has its own spatial identity rather than borrowing from the living space, and a family room large enough to function as a fully independent gathering environment.

The lifestyle return on this project at 600 to 800 square feet is described in Realm's data as the highest of any renovation category per dollar of personal enjoyment. At 800 square feet, the result is the version of that project that homeowners consistently describe as having changed their relationship with their home rather than simply improving it. You can explore more on planning a project at this scale in the Realm resource library.

The scope reality at this size that belongs in the plan from day one: Removing the rear wall of the home, foundation extension, structural engineering, roofing tie-ins, and high-end appliance and finish coordination are all standard components. Every one of them needs to be in the budget and timeline from the first planning conversation, not discovered after construction begins.

Primary Suite Addition: The Spa-Level Retreat That This Footprint Unlocks

An 800-square-foot primary suite addition is the most complete and most premium version of this project type in the entire dataset. A dressing room that functions as a genuinely distinct space rather than a large hallway closet. A spa bath with a soaking tub, a separate walk-in shower with premium fixtures and tile work, a dual vanity with real counter space, and the kind of finish details, heated floors, custom cabinetry, specialty lighting, that define the premium tier of this project category. Private outdoor access to a dedicated deck or patio adjacent to the suite makes the addition feel like a resort-quality retreat rather than an upgraded bedroom.

At 800 square feet, this addition doesn't just improve the primary bedroom situation in the home. It creates a private zone that operates at a fundamentally different quality level from the rest of the house. For homeowners who love their location and are making a long-horizon investment in daily comfort, this is the version of the primary suite addition that delivers what the project type promises at its most ambitious.

The value this addition produces in Realm's California and Washington markets is well-established and durable. The absence of a true primary suite is a quantifiable disadvantage in mid-size home appraisals, and a well-executed 800 sq ft addition more than addresses it while positioning the home at the top of its competitive tier.

The design decisions that compound over decades: At 800 square feet, the layout relationships between the dressing room, the spa bath, the bedroom, and the outdoor access are the most consequential choices in the project. Getting them right before construction begins is where the investment in thorough planning produces its most durable return. Talk to a Realm advisor about designing your primary suite addition at this scale.

Bedroom Addition: A Full Living Wing With Two Baths

A bedroom addition at 800 square feet delivers something more than two new bedrooms: a full living wing with two bedrooms and two baths that functions as a largely self-contained addition. At this scale, the two-bathroom configuration gives each bedroom genuine privacy. Both bedrooms have room for full furniture suites without clearance compromises. The addition is substantial enough that its integration with the existing home requires serious architectural thought from the first design conversation.

The market repositioning potential of this project type in California is significant. Moving a two-bedroom home into four-bedroom, two-bath territory can effectively double the addressable buyer market, because four-bedroom homes with more than one bathroom serve a meaningfully larger buyer segment. In California residential real estate, where the price gap between bedroom tiers is wider than in most markets, bedroom additions at this scale consistently produce some of the strongest resale ROI in the dataset.

What this scope involves that smaller bedroom additions do not: At 800 square feet, exterior changes to the existing home are essentially certain: roofline modifications, facade work, and structural upgrades that bring the addition into visual and structural coherence with the original structure. These are predictable components of the scope. They belong in the budget from the first conversation. Realm's advisors can help you plan a bedroom addition at this scale so the result reads as one cohesive home.

Basement ADU / Conversion: Full-Scale Below-Grade Living

Basement ADUs at 800 square feet appear primarily in Seattle projects in Realm's data, and at this size the project delivers a complete two-bedroom, one-bath apartment with full MEP buildout, egress windows, a proper separate entrance, and laundry. At this footprint, ceiling height verification becomes a specific and non-optional early-stage requirement: 800 square feet of basement livable space needs adequate ceiling height throughout the entire proposed layout, not just in the sections that happen to be taller.

The cost efficiency that defines basement conversions across all size brackets is at its most meaningful at 800 square feet because the absolute dollar savings relative to a ground-up detached ADU are largest when the footprint is largest. In Seattle's rental market, a two-bedroom basement apartment with a genuine separate entrance and well-executed finishes is a highly competitive long-term rental product.

The three assessments that determine whether the cost efficiency holds: Ceiling height throughout the full layout, egress window feasibility in both bedrooms, and waterproofing requirements for the site. Get all three answered before any design work begins. Realm's advisors can help you sequence those assessments correctly at the start of the planning process.

Finding the Project That Earns Its Cost at This Scale

Six project types at 800 square feet, all of them substantial and most of them premium, means the planning process at this scale carries more financial consequence than at any smaller size bracket. The cost floor is high across all categories, the complexity is real, and the margin for mid-project corrections is narrower than at smaller sizes. Getting the project type right before any money is spent on design or contractor outreach is the highest-value early action available to a homeowner at this scale.

For rental income: A detached ADU, garage conversion, or basement conversion each produces a premium unit at this size. At 800 square feet, the detached ADU produces the most capable and most private product. The garage conversion retains cost efficiency advantages only to the extent the scope stays within the original footprint. The basement conversion in Seattle produces the strongest cost-efficiency case when site conditions are favorable.

For personal comfort and long-term home value: A primary suite at this size produces a genuine spa-level retreat rather than an upgraded bedroom. A kitchen and family room expansion at this size produces the most complete open-concept transformation available in the dataset. Both are long-term investments in daily quality of life with well-documented secondary value in California and Washington markets.

For resale repositioning: A bedroom addition at two bedrooms and two baths is the most complete market repositioning move at this size, moving a home into four-bedroom, two-bath territory and opening it to the widest buyer pool available.

Working through these trade-offs with someone who knows your specific market and can translate your situation into an accurate scope is what Realm's advisors are there for.

Before You Budget, Understand These Numbers

The 2BR/2BA question is worth resolving early. At 800 square feet, a two-bedroom, two-bath ADU is achievable but requires a floor plan designed specifically for that configuration rather than a one-bedroom plan with a second bathroom added. Both bathrooms need adequate space alongside the bedrooms, kitchen, and living area. The layout challenge is manageable at 800 sq ft, but it requires intention from the first design session. A Realm advisor can help you evaluate floor plans for a two-bath configuration at this size.

The garage conversion cost efficiency erodes above 800 square feet. At this size, confirming how much of the scope fits within the existing garage envelope versus how much requires extending beyond it is the most important early budget question. The answer determines whether the conversion retains its cost advantage or begins to approach the cost of a ground-up build.

The marginal cost from 700 to 800 square feet is real but earns its keep. The jump in median cost from 700 sq ft ($275,000) to 800 sq ft ($338,044) is approximately $63,000, or roughly $630 per additional square foot. In a detached ADU, that increment is the difference between a spacious 2BR/1BA and a 2BR/2BA with potential rooftop access and premium finishes. In a primary suite, it's the difference between a complete suite and a full spa-level retreat with a dressing room and private outdoor access. Whether that increment is worth the cost depends entirely on your specific goals and your local rental or resale market. A Realm advisor can help you evaluate whether targeting 700 or 800 square feet makes more financial sense for your situation.

Frequently Asked Questions

How much does an 800 sq ft ADU cost to build? Based on 62 closed Realm projects at this size, the median cost across all project types is $338,044 and the average is $399,965. Detached ADUs, the most common project type at this size, typically land near the median depending on finish level, site conditions, and local permitting costs. Garage conversions that stay within the original footprint may come in somewhat below the median. Projects with rooftop decks, premium finishes, 2BR/2BA configurations, or challenging site conditions typically exceed it. Talk to a Realm advisor to get a cost estimate calibrated to your specific project type, finish level, and location.

Is a 2BR/2BA layout genuinely achievable at 800 square feet? Yes, and the project data describes it as a starter configuration at this size, meaning both bathrooms fit comfortably rather than requiring trade-offs. Both bedrooms have real clearance. Both bathrooms have proper vanity and fixture space. The kitchen and living areas function without competing for the same square footage. In all of Realm's markets, a 2BR/2BA ADU at 800 square feet is a premium rental product that commands rents at the top of the ADU category and attracts long-term tenants.

What is the average cost of an 800 sq ft home addition? Across all 62 projects at this size, the average cost is $399,965 and the median is $338,044. The gap between those numbers reflects real variation at the high end: premium primary suite additions with spa baths and private outdoor access, luxury kitchen and great room expansions with professional appliance suites, and complex detached ADUs with rooftop decks pull the average above the median. For most homeowners, the median is the more reliable planning anchor before adjusting for project type and local conditions. Browse the Realm resource library for more context on what drives cost variation at this size.

How long does it take to build an 800 sq ft detached ADU? The full timeline from initial planning to occupancy at this size typically exceeds a year when all phases are accounted for: design, permitting, contractor selection, and construction. At 800 square feet, a detached ADU is a substantial enough project that structural and MEP plan review may add time to the permitting phase compared to smaller builds. Construction alone typically runs several months once permits are in hand. The most reliable way to set an accurate timeline is to understand the specific permitting environment in your jurisdiction before committing to a target completion date, not after. Realm's advisors can help you set realistic timeline expectations for your specific project type and location.