What You Need to Know Before Starting a Home Extension in Sydney

1. Why Are So Many Sydney Homeowners Choosing to Extend?

Sydney’s housing market is brutal.
In many areas, buying a bigger home can cost upwards of $300,000–$700,000 once you factor in stamp duty, agent fees, and moving costs.
For many families, it’s cheaper and smarter to invest that money into improving what they already own.

Typical reasons people extend:

• Need extra bedrooms for growing families
• Want an open plan living/kitchen space
• Need a home office (especially post-2020)
• Planning for multi-generational living (elderly parents, adult children)
• Adding long-term property value before eventual sale

But whether you’re building a second storey or just adding 30 square metres out the back, the rules, costs, and risks are very real, and you need to be prepared.

2. What’s Considered a Home Extension? (Not Just Renovations)

Before you even start planning, know the difference:

• Home Extension:
  Physically expanding your house, adding floor area. Rear extensions, second storey additions, garage conversions.
• Home Renovation:
  Changing or improving the existing structure, kitchens, bathrooms, cosmetic upgrades.

Important:
Sydney councils treat extensions differently from renovations, with far stricter approval processes and building regulations.

3. Different Types of Home Extensions (And What You Should Know)

There’s more than one way to extend a house, but each comes with its own design, approval, and cost implications.

Rear Extensions

• Most common for Sydney homes with backyard space
• Great for open-plan kitchen/living/dining rooms
• Requires slab works, framing, waterproofing, often upgrading drainage

Second-Storey Additions

• Ideal when land size is limited
• Adds serious value (extra bedrooms, bathrooms)
• Requires stronger footings, engineering reports, possible underpinning

Warning:
If your house’s foundations aren’t strong enough, underpinning costs can blow out your budget by $30,000–$80,000.

Garage Conversions

• Turn unused garages into home offices, granny flats, or bedrooms
• Cheaper than full extensions
• Must meet BASIX (energy efficiency) requirements

Granny Flats

• Secondary dwellings (up to 60m²) with their own kitchen, bathroom, and living spaces
• Need a CDC or DA depending on the block size and zoning

Pro tip:
If you want rental income from a granny flat, talk to a certifier first, not every Sydney property qualifies easily.

4. The First Steps to Planning Your Home Extension

This is where 90% of homeowners go wrong: they rush into drawing plans before checking critical groundwork.

Here’s what you need to do first:

• Survey your land (find out property boundaries, easements)
• Get a Section 10.7 Planning Certificate from council (reveals zoning rules, heritage overlays, flood risk)
• Set your absolute max budget (including a 20% contingency)
• Decide: DA or CDC? (we’ll explain next)

Skipping these steps is why so many Sydney extensions end up delayed or abandoned halfway through.

5. DA vs CDC: How You Actually Get Approval to Build

Development Application (DA)

• Lodged through local council
• Allows more flexibility in design
• Longer timeframe (2–6 months)
• Requires public notification (neighbours can object)

Complying Development Certificate (CDC)

• Lodged through a private certifier
• Must strictly follow state rules (SEPP codes)
• Faster approval (2–6 weeks)
• No neighbour objections (if you comply)

Which is better?

• If your extension can fit under CDC rules, always go CDC.
• If your site has heritage, bushfire zoning, or flood overlays, you’ll likely need a DA.

6. Choosing the Right Designer and Builder

This is not the time to hire your mate’s cousin who “knows a bit about building.”

Good extension designers and builders:

• Know Sydney council quirks (some councils are far stricter than others)
• Have recent home extension examples you can visit
• Understand how to design to CDC rules if possible
• Offer fixed-price contracts (or staged progress payments if necessary)

Warning Signs:

• Vague quotes without itemised costs
• No evidence of previous extension projects
• Lack of insurances (Home Building Compensation Fund insurance is mandatory for projects over $20,000)

7. How Much Does a Home Extension Cost in Sydney?

Type of Extension	Average Cost (2025)
Basic Single Storey	$2,400 – $3,000/m²
Mid-Range Single Storey	$3,000 – $3,800/m²
Second Storey Addition	$3,500 – $4,800/m²
Luxury Extension	$5,000 – $6,500/m²

 Cost Drivers:

• Site access (tight sites = higher labour costs)
• Foundation upgrades
• Material selections (tile vs timber floors, stone vs laminate benchtops)
• Council requirements (stormwater detention systems can add $10,000–$20,000)

8. Common Mistakes That Destroy Extension Budgets

• Designing something that can’t be built without a DA
• Not including costs like scaffolding, asbestos removal, or upgrading old wiring
• Underestimating timelines by 2–4 months
• Not factoring in GST (if builder quotes ex-GST, it’s a nasty surprise)

9. Timeline: How Long Does a Home Extension Actually Take?

Stage	Average Time
Design and Planning	1–3 months
Council Approval	2–6 months
Construction	6–12 months

10. Hidden Costs You Need to Know About Before Starting

It’s the hidden costs, the one’s people don’t tell you about, that blow up budgets and timelines.

Here’s what you must plan for:

Upgraded Foundations

If you’re adding a second storey or heavy new structure, engineers may require foundation strengthening (underpinning).
Typical cost: $30,000–$80,000+

Stormwater Management

Sydney councils are strict about drainage. You may need to install stormwater tanks, absorption trenches, or upgrade the site’s detention system.
Typical cost: $10,000–$20,000

Asbestos Removal

If your house was built before 1990, there’s a good chance asbestos is lurking somewhere. Removal is highly regulated.
Typical cost: $5,000–$25,000

Electrical and Plumbing Upgrades

Older homes often can’t handle modern appliance loads or plumbing codes.
Typical upgrades: Switchboard replacements ($2,500–$5,000), re-plumbing sections ($5,000+)

Temporary Accommodation

If the works make part or all the house unliveable (very common during second-storey additions), you’ll need to rent elsewhere.
Typical cost: $500–$2,500/week

11. Smart Design Tips to Maximise Your Investment

If you’re investing in extending your home, do it properly. Smart design choices can deliver major lifestyle and resale payoffs.

Prioritise Open Plan Living

Modern buyers (and families) want open, flowing kitchen/living/dining spaces. If possible, design without unnecessary walls.

Maximise Natural Light

Include skylights, clerestory windows, large stacking sliding doors. Dark extensions feel small, no matter how big they actually are.

Seamlessly Blend Old and New

Match rooflines, bricks, windows and finishes. A badly blended extension can hurt resale value.

Future-Proof Your Home

• Design wider doorways and flat thresholds (good for aging in place)
• Include flexible spaces (offices that can become bedrooms later)
• Allow for solar panel installations (even if you don’t do it immediately)

Final Checklist Before Starting Your Home Extension

✅ Have you completed a land survey and site analysis?
✅ Have you checked zoning, flood risks, bushfire overlays?
✅ Have you spoken to a certifier about CDC eligibility?
✅ Have you created a realistic budget including contingency?
✅ Have you engaged a designer and builder with local Sydney experience?
✅ Have you factored in temporary accommodation if needed?
✅ Have you confirmed how approvals (DA/CDC) will be handled?
✅ Have you carefully reviewed contracts and warranties?
✅ Are you prepared for a 6–12 month journey?

If not, pause and fix these first.
It’s far cheaper to delay now than to regret it halfway through construction.

Conclusion: Plan Smart, Build Smart

A home extension can completely transform how you live, giving you the space, functionality, and value you’ve been craving.
But extensions aren’t easy. They’re complex projects that require clear planning, professional guidance, realistic budgeting, and lots of patience.

The more you understand what’s really involved, from council approvals to stormwater management to future-proofing your home, the better your results will be.

Want expert help with your home extension or renovation project in Sydney?

Talk to the team at RJG Group. We specialise in designing and building smart, beautiful home extensions that add genuine value, without the nasty surprises.

Contact us today to start planning your future space.