A custom-built guest house costs $150 to $250 per square foot, or roughly $90,000 to $150,000 for a 600 square foot one-bedroom unit, according to 2025 data from HomeGuide. That single number moves in two directions from there. How the structure gets built changes it by more than double. Where it gets built changes it by nearly as much. The rest of this article breaks down both, along with the site and utility costs that turn a per-square-foot estimate into a real budget.
Why the price does not shrink with the building
A guest house does not get a discount on any of the systems a full house needs. It requires its own foundation, its own electrical service, its own plumbing, and often its own septic or sewer connection, all of which are largely fixed costs that do not shrink much just because the building is smaller. Building a full-size house nationally runs $180 to $280 per square foot on average, according to HomeGuide, which is the same territory as the guest house figure above. That similarity is not a coincidence. A detached guest house is a complete, independent home built at a smaller scale, not a large shed with a bed in it.
Cost by build type
How the structure actually gets built is the single biggest lever on the final number. According to HomeGuide, for a 600 square foot unit:
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Custom-built, ground up: $150 to $250 per square foot, or $90,000 to $150,000 total. Takes four months to a year including design and permitting.
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Modular or prefabricated: $80 to $160 per square foot, or $48,000 to $96,000 total. Finished in one to four months.
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Converting an existing structure, such as a garage or shed: $20 to $200 per square foot, or $12,000 to $120,000 total. Usually the least expensive path, since much of the shell already exists.
What adds to the base number
Beyond the base structure, several site-specific costs commonly get missed when a homeowner first budgets a guest house. Running utilities to a detached structure costs $2,000 to $12,000, according to HomeGuide, depending on distance from the existing service. Adding a septic connection, if the existing system is not sized for the additional structure, runs $3,500 to $8,500. A concrete slab foundation runs $6 to $14 per square foot. On top of the shell, interior features add their own cost: a kitchenette runs $5,000 to $20,000, a full kitchen runs $25,000 to $45,000, and a half bath runs $4,500 to $8,000.
None of these are optional line items hiding in fine print. They are the utility and site work every detached structure needs, and they are exactly what turns a rough mental estimate into a real, considerably higher number once a contractor prices the actual site.
Guest house versus a home addition
An attached addition that adds guest space, rather than a fully detached building, often costs less overall because it can share the existing home's foundation edge, electrical service, and sometimes plumbing runs rather than requiring entirely new utility connections. The tradeoff is privacy and separation. A detached guest house gives visiting family or long-term guests a genuinely independent space, with their own entrance, their own hours, and no shared walls, which is exactly why people choose the more expensive option despite the cost gap.
How location moves the number
A 600 square foot guest house costs $70,000 to $120,000 in a lower-cost state like Michigan, compared to $125,000 to $240,000 in a high-cost state like Alaska or New York, according to 2026 data from HomeAdvisor, a swing of roughly two to one for the identical building. A sample of that state-level data:
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Michigan: $70,000 to $120,000
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Tennessee: $80,000 to $150,000
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Texas: $90,000 to $165,000
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Florida: $95,000 to $175,000
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Washington: $103,000 to $185,000
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California: $110,000 to $200,000
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Colorado: $120,000 to $210,000
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New York: $125,000 to $240,000
Oklahoma is not one of the states broken out separately in that data. A separate HomeGuide analysis of general construction costs in Oklahoma puts the state at $170 to $280 per square foot, tracking closely with neighboring Texas and landing near the middle of the national picture rather than meaningfully below it. Practically, that means the $150 to $250 per square foot range stated at the top of this article applies to Oklahoma about as directly as it applies nationally. Oklahoma is not a discount market for this kind of build. Confirm the real number against a written local bid before it goes into a budget.
A real example at guest-house scale
The Passionflower Plan is a real example of what a guest-house-scale home actually looks like built out: 497 square feet with a genuine bedroom and bathroom, a sheltered entry, and finished exterior detailing rather than a bare utility building. At this size, the design and finish decisions are the same ones a full house has to make, just at a smaller footprint.
What to check before you commit to a guest house
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Does the property's zoning allow a detached accessory dwelling, and does it cap the size or require it to match the main house's setbacks?
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Is a permit required, and how does the approval timeline compare to the rest of your project?
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Does the existing septic system have capacity for the additional bedroom, or will it need to be upsized?
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How far is the planned location from existing electrical, water, and sewer or septic connections, since distance directly drives utility cost?
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Will the guest house be used for family, long-term guests, or as a rental, since that decision affects both design and local zoning rules?
If you are weighing a detached guest house against an addition, book a design consultation and we can help you price both honestly against your actual site.
Frequently asked questions
How much does it cost to build a 600 square foot guest house?
A custom-built guest house at that size runs $90,000 to $150,000. A modular or prefab unit runs $48,000 to $96,000. Converting an existing structure runs $12,000 to $120,000, depending heavily on the existing structure's condition, according to HomeGuide.
Is a guest house cheaper to build than a house?
Not per square foot. A custom-built guest house runs $150 to $250 per square foot, close to the $180 to $280 per square foot national average for building a full house, since it needs the same foundation, utility, and permitting work at a smaller scale.
What utilities does a detached guest house need?
Its own electrical connection, water supply, and typically its own septic or sewer connection unless the existing system has capacity. Running utilities to a detached structure commonly costs $2,000 to $12,000 depending on distance from existing service.
Is it cheaper to build an attached addition instead of a detached guest house?
Often yes, since an addition can share the main house's foundation edge and utility connections rather than requiring entirely new ones. The tradeoff is that a detached structure offers genuine privacy and separation that an attached addition does not.
Do I need a permit to build a guest house?
In most jurisdictions, yes. Requirements and size limits for accessory dwellings vary significantly by county and zoning classification, so confirm with your local building department before finalizing a design.
How much does it cost to build a guest house in Oklahoma?
Oklahoma is not broken out separately in state-level guest house data, but the state's general construction costs run close to the national middle rather than below it. The same $150 to $250 per square foot range that applies nationally is a reasonable starting point for Oklahoma, confirmed against a local, written bid.
