Remodeling services
An accessory dwelling unit is a self-contained second home on a lot that already has one, with its own kitchen, bathroom, sleeping area and entrance. What separates it from an addition is independence: because it is a separate dwelling rather than more of the same house, it engages a different and more variable set of local rules, and it usually requires its own utility, fire-separation and code provisions.
HyreRemodel does not design, build or manage remodeling work. This page explains the work so you can judge a proposal and a contractor.
Settle this before anything else. Start at the planning department, not the drawing board. ADU rules are set locally within a state framework, several states have changed those frameworks substantially in recent years, and what your city allowed two years ago may not be what it allows now — in either direction. Take your parcel number and ask what is currently permitted on this specific lot: size limits, height, setbacks, parking, owner-occupancy and whether it may be rented. Every figure you find in a general article, including this one, is a description of what jurisdictions typically regulate rather than a statement of what yours requires.
The short version
The two get conflated constantly, and they solve different problems at different costs.
The first three steps are almost entirely administrative, and on ADU projects they routinely take longer than the construction.
What is permitted on this parcel right now: unit size, height, setbacks, parking, entrance location, owner-occupancy and rental rules. Free, decisive, and the step most often skipped.
Whether the sewer connection or septic system can take another dwelling, whether the water service is adequate, and whether the electrical service can carry it. Septic capacity in particular stops ADU projects outright.
A small dwelling is a demanding design problem: everything has to fit, and the size limit is usually fixed. This is where a designer earns their fee.
ADUs are permitted work everywhere, usually with stamped drawings. Some jurisdictions have streamlined review for ADUs specifically; others have not.
For a detached unit, and for any addition to an existing footprint. Access for machinery on an infill lot is a real constraint.
New framing, or reinforcing and reframing the structure being converted. Garage conversions frequently need the slab addressed and the walls brought up to habitable standards.
Water, waste, gas and electrical to the unit, across the site for a detached build. Whether services are separately metered is a decision with cost and rental consequences.
Mechanical, electrical and plumbing, plus the separation between dwellings that attached and interior units require. This is a code requirement specific to having two dwellings, not one.
As a separate dwelling, it is inspected as one. Expect the full sequence rather than a remodeling subset.
Conventional from here, with the caveat that small units reward careful storage and layout detailing more than large ones do.
Meters where separate, final inspection, and the certificate that makes the unit legally occupiable.
A second dwelling changes both. Neither is optional and both are routinely forgotten until after completion.
Cost per square foot generally rises down this list, because the amount of structure you are building from scratch rises with it.
| Interior conversion | Part of the existing house, commonly a basement or a lower level, given its own entrance, kitchen and bathroom. Cheapest where the structure and access already work. Constrained by the same ceiling-height and egress requirements as any below-grade habitable space. |
|---|---|
| Garage conversion | An attached or detached garage brought into the conditioned envelope. Foundation, walls and roof exist, which is the bulk of the saving. The constraints are insulating an uninsulated structure, raising or insulating the slab, framing the door opening, and, the one that stops projects, whether replacement off-street parking is required. |
| Attached new build | A new self-contained unit built onto the house. Shares a wall, which saves some structure but introduces fire-separation and sound requirements between dwellings that an addition would not face. |
| Detached new build | A separate small building. The most expensive per square foot because it is a complete building, foundation, envelope, roof, and its own utility runs across the site, but the most flexible, the most private, and generally the most valuable as a rental. |
What actually moves an ADU number. The utility items are the ones homeowners most often fail to anticipate.
| Factor | Why it changes the number |
|---|---|
| Conversion versus new build | By far the largest factor. An existing garage or basement provides foundation, walls and roof, typically the single biggest saving available in an ADU project. |
| Sewer or septic capacity | A second dwelling adds load. A municipal connection may need upgrading; a septic system may need enlarging or replacing, which is expensive enough to decide whether the project proceeds. |
| Electrical service and metering | Whether the existing service can carry a second dwelling, and whether you meter it separately. Separate metering costs more up front and simplifies renting considerably. |
| Distance from the house | For a detached unit, every foot of water, waste, gas and electrical trenching across the site is cost, and waste in particular needs fall. |
| Kitchen | The full kitchen is generally what makes the unit a dwelling, and it carries a kitchen’s cost and a kitchen’s trades into a small building. |
| Fire separation and sound | Attached and interior units require rated separation between dwellings. This is assembly work that an addition would not need at all. |
| Parking requirements | Where replacement parking is required, providing it can mean paving, curb cuts and losing yard, or can make the project infeasible. |
| Foundation and site conditions | Slope, soil, rock, trees and access. Small buildings on awkward sites cost disproportionately more. |
| Design and engineering | Stamped drawings are normally required. Small does not mean simple, and per square foot the design cost is higher than on a large project. |
| Impact and connection fees | Some jurisdictions levy these on a new dwelling, some waive or reduce them for ADUs specifically. This varies widely and is worth asking about explicitly. |
| Insulation and envelope on a conversion | Garages and outbuildings were generally built uninsulated with no vapour control and an uninsulated slab. Bringing them to habitable standards is most of the conversion work. |
HyreRemodel publishes no national average price for this work. Size, finish level, whether anything structural or mechanical moves, the age of the house and regional labour rates move it far enough that a single figure would mislead.
Two separate permissions, and people conflate them. Whether you may build an ADU and whether you may rent it are governed separately, and a jurisdiction can readily permit the first while restricting the second — through owner-occupancy requirements, limits on short-term letting, or conditions attached to the permit. If rental income is part of the financial case, confirm the rental rules in writing before you commit, because a unit built on an income assumption that turns out to be prohibited is an expensive guest room.
The full checklist is on choosing a remodeling contractor.
ADU rules are local, specific and currently changing faster than almost anything else in residential zoning. Drawings produced against assumed rules get redrawn. The enquiry is free and takes one visit.
These are separate permissions. Owner-occupancy conditions and short-term letting restrictions are common, and a unit built on rental income that turns out to be prohibited is a very expensive mistake to discover late.
A second dwelling adds load to a system sized for one. Where the system cannot take it, enlarging or replacing it can cost enough to end the project, which is a much better thing to know in week one.
A garage has an uninsulated slab, uninsulated walls, no vapour control and a large opening where a wall needs to be. Bringing it to habitable standards is most of the work, not a preliminary to it.
A second dwelling generally changes property tax and certainly changes insurance, and if it is rented it brings landlord obligations. None of these appear in a construction budget and all of them are permanent.
Size limits are ceilings, not targets, but the design still has to work. A unit built to the limit with a poor layout rents worse and lives worse than a slightly smaller one that is well planned, and the cost difference between them is mostly design, not construction.
Independence, and specifically that it is a self-contained dwelling. The practical test most jurisdictions apply is whether it has its own kitchen, along with its own bathroom, sleeping area and entrance. A bedroom and bathroom suite added to a house, however separate it feels, is generally still part of the same dwelling; add a kitchen and it usually becomes a second one. This matters because a second dwelling engages a different set of rules: occupancy, fire separation between units, utility connections, parking and sometimes owner-occupancy requirements, none of which apply to an addition.
Because they are local, and because this is the area of residential zoning that has changed most in recent years. Several states have passed framework legislation intended to make ADUs easier to build, cities have amended their ordinances in response, and some of those statutes have themselves been reorganised and renumbered since. Any specific figure published in a general article has a short shelf life and is wrong in a way a reader cannot detect. Your planning department knows what applies to your parcel today, the enquiry is free, and it is the only source that is current by definition.
Usually significantly, because the foundation, walls and roof already exist and no new footprint is created. What tempers the saving is that a garage was built as a garage: the slab is uninsulated and often sloped to drain, the walls and roof are typically uninsulated with no vapour control, there is a large opening where a wall now needs to be, and the electrical provision is minimal. Bringing all that to habitable standards is the substance of the conversion rather than a preliminary to it. The other thing to check first is parking — many jurisdictions require a certain amount of off-street parking to be maintained, and where replacement parking cannot be provided the conversion may simply not be permitted.
Sometimes, and it is a separate question from whether you may build one. Jurisdictions commonly attach conditions: an owner-occupancy requirement that the property owner live in one of the two units, restrictions or prohibitions on short-term letting, or limits on the number of occupants. Some places have removed such conditions in recent years; others have added them. If rental income is part of the reason for building, get the rental position confirmed by the planning department in writing before committing money, because it is the assumption most likely to be wrong and the most expensive one to be wrong about.
Generally yes, because it adds a dwelling and therefore assessed value, and the permit is normally what triggers reassessment. How much depends entirely on local assessment practice and rates, so your county or municipal assessor is the only reliable source. Insurance also changes: a second dwelling, and particularly a rented one, is a different risk from a single-family home and needs to be declared. Both are ongoing costs that belong in the financial case alongside the construction budget, and both are routinely left out of it.
It depends on your jurisdiction and your utility, and where you have a choice it is worth thinking about rather than defaulting. Separate metering costs more up front — a second service, a second meter, and sometimes a service upgrade, but it makes renting considerably simpler, because the tenant pays their own usage directly and you are not apportioning a shared bill. Shared metering is cheaper to install and fine where the unit houses family. The sewer and water connection is a separate question again, and in some places a second dwelling triggers a connection fee whether or not it is separately metered.
Where two dwellings share a structure (an attached ADU, or one created inside the existing house) building codes generally require a rated separation between them: an assembly of specified materials designed to resist the spread of fire from one dwelling to the other for a defined period, and often to limit sound transmission as well. It is a requirement that follows from there being two dwellings rather than one, which is why an addition of the same size does not face it. The specific assembly required comes from the code your jurisdiction has adopted, and it is one of the things a designer has to resolve on the drawings rather than a detail to be worked out on site.
The construction is often shorter than people expect and the approvals are often longer. A small detached unit is a modest building; getting to the point of being allowed to build it involves planning enquiry, utility feasibility, design, engineering and plan review, and in many jurisdictions that front end dominates the schedule. Some places have introduced streamlined or time-limited review for ADUs specifically; others have not. Ask your planning department what the current review timeline is for an ADU application — it is one of the few schedule figures you can get authoritatively before you start.
That depends on your local rental market, your construction costs, your financing and the rules on renting it, and those vary enough that a general answer would be worthless. What can be said usefully is which assumptions carry the most risk: that renting will be permitted on the terms you expect, that the septic or sewer connection will take a second dwelling without major work, and that the construction cost of a small building is proportionate to its size, which it typically is not, because a small dwelling still needs a kitchen, a bathroom, a foundation, an envelope and its own utility runs. Test those three before the spreadsheet, not after.
No. HyreRemodel is an independent information resource and a platform intended to connect homeowners with remodeling contractors. We do not design, engineer, build, project-manage or supervise construction, we do not employ tradespeople, and we do not warrant anyone who does. We also do not give legal or zoning advice — what is permitted on your parcel is a question for your planning department. This page exists so you know what to ask them and what to ask a builder.
Information on this page is general and may vary by location, building code, zoning, property condition and individual circumstances. It is not legal, design or engineering advice. HyreRemodel is an independent remodeling resource and connection platform, not a remodeling contractor, and does not perform, supervise or warrant construction work.
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