Our Past Projects
See our past designs & projects
Explore first floor additions and home extensions we've completed in Sydney — adding space, style, and long-term value.
Sydney's Trusted Home Builders
Reimagine what your home can be.
It’s time to expand your home the smart way. Kastell Building Co. specialises in ground floor extensions and first floor additions that maximise liveable space, boost your home’s value, and suit the way your family lives - all with expert planning, design support, and transparent pricing.
🧭 Uncover what’s possible with your block
📐 Work with a builder who understands flow
📊 Compare extension options for your budget
📝 Get a detailed, fixed-price proposal
Pricing Guidelines
Extension & Addition pricing guidelines
Get a clear picture of what’s achievable at different budget levels — from internal reconfigurations to full second-storey additions. Our guides show typical inclusions to help you plan with confidence.
Modern Rear Extension
Starting from
$330,000
As a guide this could include:
✅ Extend open-plan living area
✅ Add large stacker or bifold doors
✅ Covered alfresco with ceiling fan
✅ Polished concrete or timber floors
✅ New slab and structural works
✅ Energy-efficient glazing
✅ Internal painting and lighting
✅ Seamless indoor-outdoor flow
✅ CDC or DA approvals included
✅ HBCF insurance and handover
First Floor Home Addition
Starting from
$420,000
As a guide this could include:
✅ Add 2x bedrooms and 1x bathroom
✅ New internal staircase
✅ Timber or hybrid flooring
✅ Full roof and external cladding
✅ Lighting, power, and data points
✅ Custom built-in wardrobes
✅ Double glazing to all windows
✅ Full interior painting
✅ DA or CDC approval & engineering
✅ HBCF warranty and certifications
Ground Level Space Extension
Starting from
$220,000
As a guide this could include:
✅ Add guest bedroom or home office
✅ New bathroom or powder room
✅ Rear or side layout extension
✅ Upgrade flooring in new areas
✅ Split system air-conditioning
✅ Joinery or built-in robes
✅ New roof and gutters (if required)
✅ Approvals and site prep included
✅ HBCF cover and warranties
✅ Minimal disruption construction
Why build with Kastell?
Minimal Disruption Guarantee
We keep your home liveable throughout the build, with site protection, dust control, and a clean-as-you-go team who respect your space and routine.
Built for Long-Term Value
From joinery construction to appliance choices, we focus on durable materials and timeless design so your kitchen stays functional and stylish for years.
Quotes with Line-Item Detail
No lump sums, no grey areas. Our quotes break down exactly what you’re paying for - so you can cut, upgrade, or swap with full visibility.
Spec With Confidence
We’ll guide you through selections with real-world advice - helping you choose fittings that won’t date, break, or blow out your budget.
Clean Site Promise
We leave your home liveable during the build - with daily site cleans, dust control measures, and respectful tradies who treat your space like their own.
Pro-Level Project Tracking
You’ll get full access to your project’s live schedule, updates, and photos - so you’re never chasing us for answers. Our systems are built to keep you in control.
Our Trusted Partners
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Our Process
Extension & Addition process
Planning
We explore your space, goals, and budget to map out what's possible and worth investing in.
Maximising ROI
We help you make smart decisions on where to spend for the biggest impact and long-term value.
Design Selections
Together we choose layouts, finishes, and fittings that suit your lifestyle and design taste.
Quoting & Prep
You'll receive a detailed fixed-price contract and we prep the site for a safe, clean start.
Construction
Our in-house team delivers your renovation to spec then walks you through every last detail.
Get Inspired
Ideas & Inspiration for your project
Browse our Pinterest boards filled with extension layout inspiration, external finishes, and interior design ideas for your growing home.
Get a no-obligation Quote
Ready to Extend? Let's chat
Request a free renovation consultation to explore layout options, budget expectations, and project feasibility with a licensed Sydney builder.
FAQs
Extension FAQs for Sydney Homes
Answers to common questions on house extension costs, approvals, timelines, and what’s involved in a first floor addition project.
How much does a first-floor addition cost in Sydney?
The cost of a first floor addition in Sydney typically ranges from $250,000–$450,000+, depending on size, complexity, and finishes. Learn more in our first floor extension cost guide. For a tailored quote, book a free renovation planning call with Kastell Building Co., your trusted first floor extension builders.
Do I need council approval for a home extension in NSW?
Yes, most home extensions in Sydney require approval via either a DA (Development Application) or a CDC (Complying Development Certificate). Learn more about which applies to your project in our blog: DA vs CDC – What You Need to Know or visit the NSW Planning Portal for official guidelines.
How long does a first-floor addition or extension take?
Timelines vary based on project size and approval method, but most first floor additions Sydney take 6–9 months from design to completion. Learn more about the timeline for a home extension in Sydney and how Kastell keeps things moving efficiently.
Can I live in my home during the extension build?
In many cases, yes. Kastell works with you to stage construction and maintain safety. However, for major second storey additions Sydney, a temporary move may be recommended. Learn more during your free consultation.
What’s the difference between a DA and a CDC?
A DA (Development Application) is assessed by your local council, while a CDC (Complying Development Certificate) is a fast-tracked approval via a private certifier. Both can be used for home additions, depending on your property and plans. Learn more in our blog: DA vs CDC.
Will a home extension increase my property value?
Absolutely. A well-designed home extension in Sydney can significantly increase your resale value and usable space. Learn how home additions impact ROI and why many clients choose Kastell to add a second storey Sydney.
What factors affect the cost of a home extension?
Key cost drivers include size, materials, access, heritage restrictions, and structural changes. For a deep dive, explore our cost planning expertise or use a cost calculator to estimate your project. Kastell provides transparent quotes tailored to your scope.
How do I choose the right builder for a home extension?
Look for licensed builders in Sydney with clear communication, proven experience with custom house extensions, and strong reviews. Kastell Building Co. guides you from concept to completion, offering transparency, education, and quality workmanship you can trust.
Testimonials
Real client feedback on extensions
Hear what Sydney homeowners say about working with Kastell for their home additions - from design to delivery and everything in between.
"We're so happy with the space Kastell created. The new upper level fits our lifestyle perfectly and blends beautifully with our home."
Ayesha & Daniel
First Floor Addition, Lane Cove
"The Kastell team made the whole process easy. The result is a bright, open space that feels like it's always been part of the house."
Aoife & Jordan
Home Extension, Neutral Bay
"From planning to build, everything was handled professionally. We finally have the space our family needed without moving!"
Leila & Samir
First Floor Extension, Mosman
Blog
Extension Planning Advice
Browse renovation blogs covering first floor additions, DA vs CDC, design tips and how to add long-term value to your Sydney home.
Free Resource
9 secrets for a stress-free renovation
Your essential guide for renovating in Sydney. Avoid costly mistakes and stress in your next renovation – Get our free guide now!
