Written by JSW Plumbing | Published on June 2025
A new bathroom typically requires rough-in drain work, hot and cold water supply lines, a vent stack connection, and fixture installations including a toilet, basin, and shower or bath. The exact scope depends on whether you are adding a bathroom from scratch or converting an existing space. This guide covers every bathroom plumbing element you need to plan for, with typical costs and local considerations for homeowners across Hornsby, Thornleigh and the Hills District.
Understand What Rough-In Plumbing Actually Means
Rough-in plumbing is everything that happens inside your walls, floors, and ceiling before any fixtures are installed. It is the foundation of your bathroom and the most labour-intensive part of the job. Getting this stage right means your shower drain, toilet, and basin all sit exactly where they should.
Rough-in work for a new bathroom generally includes:
- Cutting into existing drain lines or running new drain pipes to the main sewer
- Installing waste pipes for the toilet, basin, bath, and shower at the correct fall (typically 1:60 gradient under Australian plumbing code)
- Running hot and cold copper or PEX water supply lines to each fixture location
- Connecting to or extending your vent stack so drain odours cannot enter the home
- Installing floor waste drains in wet areas
In older homes in Thornleigh and surrounding suburbs, existing drain pipes are often terracotta or cast iron. These may need to be partially replaced before new PVC connections can be made. Your plumber will assess this during the planning stage.
Rough-in plumbing must be inspected and approved before walls are closed up. In NSW, this work must be completed by a licensed plumber, and a compliance certificate is required.
Know the Plumbing Requirements for Each Fixture
Each fixture in your bathroom has its own plumbing requirements. Understanding these upfront helps you plan your layout and avoid expensive changes later.
| Fixture | Drain Size Required | Water Supply | Special Requirements |
|---|---|---|---|
| Toilet (WC) | 100mm | Cold only | S-trap or P-trap, correct setout distance |
| Basin / Vanity | 40mm | Hot and cold | P-trap under cabinet or in wall |
| Shower | 50mm minimum | Hot and cold | Floor waste, waterproofing, correct fall |
| Freestanding Bath | 40-50mm | Hot and cold | Floor-mounted tap connections or wall set |
| Built-In Bath | 40mm | Hot and cold | Overflow connection required |
| Laundry Tub (ensuite) | 40mm | Hot and cold | Trap required, check local council |
The toilet setout measurement is particularly important. This is the distance from the finished wall to the centre of the drain. Standard Australian setout is 140mm to 160mm. Getting this wrong means your chosen toilet pan may not fit correctly, which is a costly fix after tiling.
Plan Your Hot Water Supply for the New Bathroom
Adding a new bathroom increases demand on your existing hot water system. Before committing to your bathroom layout, you need to check whether your current system can handle the extra load.
A standard shower uses around 9 to 15 litres of water per minute. If your household already runs multiple showers in the morning, adding another outlet without upgrading your hot water system can mean cold showers for whoever goes last.
Key questions to answer before rough-in starts:
- What is the capacity of your current hot water system in litres?
- How far is the new bathroom from the hot water unit? Long runs cause delays and water wastage.
- Is your system gas, electric, or heat pump, and does it have room to expand?
- Would a point-of-use electric unit or a recirculating pump make more sense for the distance?
Many homes in the Hills District have solar or heat pump hot water systems. These are generally efficient, but the tank capacity is fixed. JSW Plumbing can assess your existing system during the quoting stage and recommend whether an upgrade or a secondary unit is needed.
Compare Typical Plumbing Costs for a New Bathroom in the Hills District
Plumbing costs vary depending on the location of the new bathroom, the complexity of the rough-in, and the fixtures you choose. Below are typical ranges for the Hornsby, Thornleigh and Hills District area based on current market rates.
| Plumbing Scope | Estimated Cost (AUD) | Notes |
|---|---|---|
| Rough-in plumbing only | $3,000 – $6,000 | Depends on access and run length |
| Fixture installation (toilet, basin, shower) | $800 – $1,800 | Excludes fixture purchase price |
| Full bathroom plumbing (rough-in + fixtures) | $4,500 – $9,000 | Standard 3-fixture bathroom |
| Adding ensuite above existing bathroom | $3,500 – $7,000 | Easier drain stack access |
| New bathroom on slab (no existing drains nearby) | $6,000 – $12,000+ | Core drilling and longer runs |
| Hot water system upgrade | $1,200 – $3,500 | Depends on system type and size |
These are estimates only. The best way to get an accurate figure is to have a licensed plumber walk through the site. JSW Plumbing offers on-site assessments across Thornleigh, Hornsby, Cherrybrook, Castle Hill and surrounding areas before any commitment is required.
Labour rates in NSW typically sit between $100 and $150 per hour for licensed plumbers. A full bathroom rough-in usually takes two to three days depending on complexity.
Understand Venting and Why It Matters
One of the most overlooked parts of bathroom plumbing is the venting system. Every drain in your bathroom needs to be vented to prevent siphoning of the water seal in your traps. Without proper venting, you get gurgling drains and sewer gas smells inside the home.
In Australia, venting requirements are set out in AS/NZS 3500, the national plumbing and drainage standard. In NSW, all licensed plumbers must follow this code. Your new bathroom drains must connect to an existing vent stack or have a new vent pipe run to the roof.
If your new bathroom is on the ground floor and relatively close to existing drain lines, connecting to the existing vent stack is usually straightforward. If it is on an upper floor or on the opposite side of the house, your plumber may need to run a new vent pipe, which adds to the cost and ceiling/wall work involved.
Air admittance valves (AAVs) are sometimes used in situations where running a vent to the roof is not practical. These are approved in certain configurations under Australian standards, but your plumber will confirm whether they are appropriate for your specific layout.
Check What Approvals and Certificates You Need in NSW
Adding a new bathroom in NSW involves more paperwork than most homeowners expect. Skipping approvals can cause problems when you sell your home or make an insurance claim, so it is worth understanding the requirements upfront.
Here is what is typically required for new bathroom plumbing in NSW:
- Licensed plumber: All plumbing work must be carried out by a plumber licensed under NSW Fair Trading. Unlicensed work is illegal and can void your home insurance.
- Compliance certificate: Your plumber must issue a Certificate of Compliance for Plumbing and Drainage work once the job is complete. This is a legal requirement in NSW.
- Development approval (DA): Adding a new bathroom may require a DA or a Complying Development Certificate (CDC) depending on the scope of the building work involved. Check with Hornsby Shire Council or The Hills Shire Council before starting.
JSW Plumbing handles the plumbing compliance certificate as part of every bathroom plumbing job. We work alongside builders, tilers, and electricians to coordinate inspections and keep your project on schedule.
Choose the Right Fixtures for Your Plumbing Layout
The fixtures you choose need to match the plumbing layout, not the other way around. Many homeowners pick a toilet or basin they love online, only to find the drain connection does not suit the rough-in position. Plan the plumbing first, then select your fixtures.
Things to confirm before purchasing fixtures:
- Toilet trap type: S-trap goes through the floor, P-trap goes through the wall. Your rough-in determines which you need.
- Toilet setout distance: Measure from the finished wall (including tiles) to the drain centre. Most Australian toilets are designed for 140mm to 160mm.
- Basin waste position: Wall-hung basins need the waste in the wall. Vanity basins drain through the cabinet floor. Confirm before rough-in.
- Shower head type: Rail shower, overhead rain shower, and wall-mounted units all have different connection heights. Tell your plumber which style you want before pipes are set.
- Bath tap style: Floor-mounted taps need separate floor connections. Wall-mounted taps need wall supply rough-ins. These cannot easily be swapped after tiling.
If you are working with a bathroom designer or a specific tile layout, share those plans with your plumber early. Moving a drain or supply point after tiling is expensive and often means removing tiles. Good coordination upfront avoids this entirely.
FAQs on New Bathroom Plumbing
Can I add a bathroom anywhere in my house?
Technically yes, but the further the new bathroom is from your existing drain stack, the more complex and costly the plumbing becomes. Ground floor bathrooms near existing drains are the most cost-effective. Upper floor additions and slab-on-ground layouts require more planning and budget.
How long does rough-in plumbing take for a new bathroom?
Rough-in plumbing for a standard three-fixture bathroom typically takes two to three days. This includes cutting in drains, running supply lines, and connecting to the vent stack. Difficult access or longer pipe runs can extend this to four or five days.
Do I need council approval to add a bathroom in NSW?
It depends on whether any structural work is involved. The plumbing itself does not require a DA, but if you are adding a room or modifying the building structure, you may need a DA or CDC from Hornsby Shire Council or The Hills Shire Council. Your plumber can advise on the plumbing compliance side, and a builder or certifier can guide you on any building approvals needed.
What is the difference between a P-trap and an S-trap toilet?
An S-trap drains through the floor and is the most common type in Australian homes. A P-trap drains through the wall and is used in situations where the floor cannot be penetrated, such as on a concrete slab or upper floor. Your plumber will determine which suits your bathroom layout during the rough-in stage.
Can my existing hot water system handle an extra bathroom?
It depends on the capacity of your current system and how many outlets already use it. A licensed plumber can assess your system and advise whether an upgrade or a secondary hot water unit is needed. This is worth checking before rough-in so any upgrades can be planned into the project.
How much does it cost to plumb a new bathroom in the Hills District?
Full bathroom plumbing including rough-in and fixture installation typically costs between $4,500 and $9,000 in the Hornsby, Thornleigh and Hills District area. Complex layouts, slab penetration, or long pipe runs can push costs higher. An on-site quote gives you the most accurate figure for your specific home.
Wrap Up and Next Steps
Adding a new bathroom is a significant investment, and the plumbing work is the part that must be done right before anything else happens. From drain rough-ins and vent connections to fixture selection and hot water capacity, every decision affects the next one.
JSW Plumbing works with homeowners across Hornsby, Thornleigh, Castle Hill, Cherrybrook and the wider Hills District to plan and complete new bathroom plumbing from the first pipe to the final fixture. We handle the compliance certificate so you are covered from day one.
Ready to get started? Contact JSW Plumbing to arrange an on-site assessment and quote for your new bathroom. Call us or send a message through our website and we will get back to you promptly.