Convert existing, save 40% vs new build
Garage conversion to granny flat.
If you have a detached or semi-detached garage 25m²+, conversion to habitable Class 1a dwelling is often the cheapest pathway to a granny flat. $95K–$135K vs $175K–$220K for a new build. Here’s the framework.
When conversion makes sense.
Conversion is the right pathway if:
- Existing garage is detached OR semi-detached with shared wall
- Size is at least 25m² (smaller becomes unworkable)
- Existing structure is sound (concrete slab in good condition, walls plumb, roof intact)
- Block has adequate setbacks under Territory Plan
- Crown Lease permits secondary residence
- Existing services (sewer, water, power) are close enough to extend
If any of these fail, new build is the better path.
The eight-stage conversion.
- Structural assessment. Existing slab to AS 2870, wall framing to AS 1684, roof structure load review. Engineer report: $1,400.
- Insulation retrofit. R5.0 wall (often blow-in cellulose or rigid panel internal), R6.0 ceiling. Vapour barriers and thermal-break detailing. $4,800–$7,200.
- Window upgrade. Existing garage windows replaced with double-glazed thermal-break. Garage doors removed and walled-in or converted to French doors. $5,400–$8,200.
- Slab finishing. Concrete grinding + polish, or screed + tile, or carpet underlay. Convert from utilitarian garage floor to habitable surface. $3,200–$5,400.
- Service connections. Sewer + water extended from main dwelling, separate sub-meter for electricity. Connection fees + plumbing: $6,800–$11,400.
- Bathroom + kitchenette installation. Plumbing rough-in, fixtures, joinery, splashbacks. $14,200–$22,400.
- Heating system. Reverse-cycle inverter AC or in-slab hydronic. $3,800–$5,800.
- EER 6-star certification. Final NatHERS modelling + Building Approval issued by Private Surveyor. $1,800–$2,800.
Typical conversion cost breakdown.
32m² existing detached garage in Belconnen, convert to studio + bathroom + kitchenette:
- Demo (garage door, internal lining): $1,800
- Slab grinding + polish: $2,400
- Wall + ceiling insulation retrofit: $5,400
- Double-glazed windows (3 new) + French doors: $7,200
- Internal walls + plasterboard: $4,800
- Kitchenette joinery + appliances: $9,400
- Bathroom (compact 3m²): $11,200
- Plumbing connection from house: $5,800
- Electrical (sub-meter + circuits): $4,400
- Reverse-cycle inverter AC: $3,800
- Floor coverings (carpet bedroom area): $1,200
- Painting + finishes: $2,600
- NatHERS + Building Approval: $2,640
- Total: $62,640 incl. GST
Net $62K compared to $175K–$200K for a new-build studio. $113K cheaper. Pays back even faster as a rental investment.
When conversion doesn’t work.
We’ll honestly tell you when new build is better:
- Existing slab is cracked, settled, or unsuitable. Demolition + new slab often $8K+ — new build becomes competitive.
- Garage is attached to dwelling. Fire-separation requirements add complexity. Sometimes blocked entirely.
- Existing roof is asbestos or in poor condition. Asbestos removal alone is $4–$8K. New build with new roof preferred.
- Garage is under 25m². Final habitable space too small to be commercially viable.
- Crown Lease prohibits secondary residence in this form. Some Inner-South leases have explicit conditions.
Where we work.
Free garage conversion assessment.
30 minutes on-site. We’ll tell you straight: convert or build new.