Converting Your Garage Into A Granny Flat? Here’s what you need to know

House   Brett Rowley  | 18 November 2019

Deciding to convert your garage into a habitual space is no easy task. Where do you even begin? Unfortunately, it’s not just a matter of throwing in a couch and television, although we can see the appeal. 

When it comes to converting your garage into a granny flat you need to consider what’s actually missing, let’s compare it to your home — 

  • Insulation and interior cladding
  • Interior walls
  • Ceiling, insulation and cladding
  • Flooring
  • Plumbing and electricity
  • Heating and cooling
  • Ventilation
  • Lightning
  • Waterproofing
  • Natural light

If you’re going to convert your garage into a granny flat, it’s also going to need a toilet, bathing facilities and a kitchen for cooking and food storage.

And these are just the basic requirements. You’ll also want to make your garage more comfortable and attractive than a slab of cold concrete and four walls by adding flooring, ventilation, lighting and windows. 

You’ll definitely need some form of heating and cooling for those extremely hot days and cold winters, especially in Australia where temperatures can reach well over 40 degrees.

Seek council approval

Next, you’ll have to seek council approval since you’re converting a non-habitual space into a habitable room, and apply for a building permit. Depending on your location, local councils may have different requirements, so it’s best to get in contact with them. 

There are two types of applications for a building permit:

  • Certified applications — Applicant has already engaged a private building surveyor to check plans and specifications. Have completed a certificate of design which confirms the proposal complies with building standards and may take up to 10 business days for a decision to be made.
  • Uncertified applications — Submitted directly to the city without a certified application and an independent building surveyor.

Your granny flat will need to comply with the National Construction Codes – Building Code of Australia (NCC BCA) as well as meet the requirements of the Residential Codes (R-Codes) or Residential Development Local Planning Policy.

Such provisions can include:

  • The walls of the new addition must maintain a continuous cavity with the existing dwelling
  • The interior floor to ceiling height must be a minimum of 2.4m
  • Since garage floors are not elevated, you’ll need to consider how the proposed works will prevent the ingress of water into the new room, this is where you’ll need a licensed plumber/drainage expert.
  • In addition, a licensed plumber will need to carry out all water-proofing and plumbing works, and provide a water-proofing certificate for all new wet areas.
  • A plumber will also need to ensure your granny flat isn’t going to affect the sewer mains, and contact your local council.
  • The existing slab may also need to be drilled to install a termite barrier
  • A licensed electrician must install all lighting, power, stove and air conditioning, plus a phone line.
  • Natural light and ventilation must be maintained/installed.
  • Hard wired smoke detectors must be installed to protect all existing and proposed bedrooms of the house
  • Any existing power metre box within the proposed enclosure must be relocated to an external wall

Note: With recent changes to the legislation Australia-wide, granny-flats can now be built in all residential zones as long as there is no more than one main home and one granny flat on the same property. The size of the granny flat can be no more than 70m squared, or the maximum allowed by your local council.

You can find out more information about ancillary dwellings on this fact sheet.

How much does it cost to convert a garage into a granny flat?

In Australia, garage conversions can cost anywhere between $6,000 to $20,000 with an average of around $12,000. Depending on the type of room you plan to convert it to, the price may vary.

  • Living Room conversion cost: $5,000 – $10,000
  • Apartment or Guest House conversion cost: $15,000 – $30,000
  • Kitchen conversion cost: $6,000+
  • Granny flat cost: $3,000 – $30,000
  • Utility room (home gym) cost: $6,000+
  • Home office/work space cost: $5,000

Renovation cost breakdown

Conversion permit

Based on your location, you may need to purchase a permit from your local authority, which can cost between $1,000 to $1,500. If you’ve got a contractor, they will be able to inform on the costs and process.


If you plan on converting your garage into a granny flat it’s going to need insulation. You can expect to pay around $1 per square foot on insulation.

Your contractor should be able to tell you what’s suited to your needs, where it should go and how much you’re going to need.

There is also the option of getting an insulated garage door or installing insulation panels onto your existing door. 

Interior walls

To keep your remodelled room as comfortable as the rest of your home, you’ll need to consider drywall hanging, which can cost around $1,700 to $2,000.


If your garage doesn’t already have windows, you’ll need to add in at least one, especially if you plan to use it as a bedroom or rent it out. Expect to pay around $1,000 per window. You’ll also need an egress window that’s big enough for a person to escape through in case of a fire, which has an average cost of $3,500.

Heating & cooling

These days, air conditioning has become an essential in the Australian housing construction industry, with most family homes equipped with some form of heating/cooling device. Costs for air conditioning systems installation in Perth such as ducts and vents average between $500 to $2,000. 


Most garages should already have some form of electricity, whether it be a lightbulb or an automatic garage door. A granny flat is going to require electricity to power the kitchen and creature comforts and may Cost between $75 to $500.

Remember, it’s illegal to conduct any sort of electrical work in WA, so you’ll need to hire a licensed electrician to add more wiring and power outlets for additional devices. 

Phone/internet connection

Expect to pay around $300 to $500 to install a wireless network and additional outlets.

Interior design

Whether you choose to design yourself or hire a professional, you can expect to pay anywhere between $2,000 to $11,000, depending on the type of materials and finishings you choose. This can include:

  • Flooring cost $1,500 to $4,500 — flooring types: luxury vinyl, hardwood or cement, carpet and laminate.
  • Bedroom inclusions — bed, side tables, decorations, wardrobe and nightstand
  • Bathroom inclusions — bathtub or shower, basin, vanity, and a mirror
  • Kitchen inclusions — fridge, sink, countertops (granite, marble or composite), cabinetry
  • Garage door installation can cost approximately $40 to $70 per hour depending on the amount of work required.

How your garage door can help

A well-designed and constructed customised garage door can provide adequate ventilation, natural lighting, insulation, and can be sealed with waterproofing materials.

Most granny flats will usually opt to remove the garage door as it is no longer required, but can be kept in other renovations or replaced with a custom garage door with window inserts to bring some natural light in the room. Speak to a friendly professional at Gryphon Garage Doors Perth to see how your garage door can benefit your renovation.

Brett Rowley

Brett is a garage door expert who also runs Gryphon Garage Doors. He strives for great customer service, excellent product knowledge and constant innovation.