How Much Does It Cost To Build A Detached Mother-In-Law Suite?

As housing prices continue to rise, more and more homeowners are looking for creative ways to add value to their property. One of the most popular options is building a detached mother-in-law suite.

How Much Does It Cost To Build A Detached Mother-In-Law Suite

Having an additional living space in your backyard can provide you with extra income or even just a place for family members to stay when they visit. 

But how much does it cost to build one (see also “How Much Does It Cost To Build A 12×12 Screened-In Porch?“)? In this article, we’ll explore what goes into the cost of constructing (see also “Are Construction Costs Going Down?“) a detached mother-in-law suite and give you some tips on how to save money as well.

Cost Factors

The primary factor affecting the cost of building a detached mother-in-law suite is the size (see also “How Much Does It Cost To Build A Duplex?“). The larger the space, the more materials and labor needed to construct it, which drives up construction costs.

On average, to build a self-contained detached suite, expect to pay around $83,000 – $130,000. 

Other factors include any necessary permits or zoning restrictions that may require additional engineering fees and inspections. Additionally, the type of finishes chosen for interior decor, appliances, plumbing fixtures and other items all contribute to the overall cost.

When considering what kind of investment will be required for a new mother-in-law suite, you should also consider long term maintenance costs such as regular landscaping upkeep and utilities like electricity and water. 

Moreover, if there are plans for adding an extra bedroom (see also “How Much Does It Cost To Add A Bedroom To A House?“) or bathroom in the future, this too should be factored into budgeting considerations since these can add significantly to total expenses.

So before starting a project like this, you should carefully weigh all relevant factors when determining how much money needs to be set aside for completion of the work.

Converting A Finished Room

Converting A Finished Room

The cost of converting a finished room into a detached mother-in-law suite depends on the existing condition of the space and any upgrades needed.

For example, if the room already has plumbing and electricity, then it might be more affordable than if those elements need to be installed from scratch.

Here is a bullet point list of items that are usually required for this type of conversion:

  • Electrical wiring updates or installation
  • Plumbing fixtures and pipes installation
  • HVAC system additions or enhancements
  • Drywall replacement or repairs
  • Flooring replacements or modifications

Making sure all these systems work together properly can involve professional assistance, which will add to the total costs.

In addition, consider other factors like furniture purchases, decor installations and any additional construction needs before making your final budget.

Building An Adu

Depending on the size and complexity of the ADU, you may be looking at thousands or even tens of thousands of dollars in construction costs alone.

This is because an ADU requires additional permits, materials, and labor than the average home renovation project. 

Additionally, if your local zoning laws or regulations require special features such as fire sprinklers or high energy efficiency ratings for your new space, these will further add to the total cost.

Of course, there are some ways to save money during the construction process. For instance, many homeowners opt for prefabricated modules instead of constructing from scratch; this helps reduce both time and material costs. 

It’s also possible to take on certain aspects of the build yourself – provided you have sufficient knowledge and experience – which can help shave off a significant portion of expenses.

Converting A Garage

Converting A Garage

The cost of building a detached mother-in-law suite largely depends on the type of conversion being done (see also “How Much Does It Cost To Build A Barndominium?“). If you plan to convert an existing structure such as a garage, there are several considerations that must be taken into account:

1. Cost of materials and labor: Converting any unoccupied area requires materials (e.g., drywall, insulation) and professional labor for installation. This can range from $4-$15 per square foot based on the complexity of the project.

2. Electrical wiring: Updating electrical wiring is critical for safety and convenience in your new space. You may need additional outlets and higher voltage capability depending on what appliances will be used in the space.

Labor costs for this typically start at about $100/hour, with parts costing extra.

3. Plumbing: Depending on if you’re installing running water and sewage lines or simply adding toilets or sinks to existing plumbing, the cost can vary greatly from DIY projects ($200+) to more complex installations requiring licensed plumbers ($750+).

4. HVAC system: Heating and cooling units should match the size of your new room while providing efficient energy usage across all seasons; these systems usually cost around $2,000+.

General Contractors

General Contractors

Finding the right contractor to build your detached mother-in-law suite can be a daunting task. You’ll want someone who is experienced, reliable and has an excellent reputation in the community.

Before selecting a general contractor, you should do some research on different companies and get at least three bids for the project.

Be sure to ask potential contractors questions about their experience and qualifications for building such structures.

Inquire about any restrictions that may apply for constructing something like this on your property, as well as whether they will need permits from local authorities or not.

Also, make sure to discuss payment terms before starting work, so there are no surprises when it comes time to pay them.

Additional Costs

To build a detached mother-in-law suite, there are additional costs beyond the initial contractor fees. Depending on the size and complexity of the project, these costs can quickly add up. Here’s a look at what to expect when you’re budgeting for your new addition:

  • Building materials: Materials such as lumber, drywall, roofing shingles, insulation and more will need to be purchased in order to construct the mother-in-law suite.
  • Labor costs: This includes both skilled labor (for example, electricians or plumbers) and unskilled labor (carpenters or general laborers).
  • Permits and inspections: You may need various permits from local authorities before construction begins. Additionally, city inspectors must approve certain aspects of the work during different phases of construction.
  • Finishing touches: Don’t forget about furnishings and decorations! From furniture to paint colors, make sure you factor in those extra expenses that will make this room feel like home.


For anyone considering this kind of project, it’s important to do your research and get multiple quotes from different contractors before starting construction.

It’s also wise to factor in any additional expenses that may come up during the process, so you don’t end up having to pay more than expected.

With careful planning and budgeting, I’m sure you’ll find the perfect solution for adding extra living space without breaking the bank!

Luke Powell
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