Why two bathroom quotes can differ by ten thousand dollars

You get three quotes for the same bathroom. One is $22,000, one is $27,000 and one is $31,000, for the same room, in the same house, from the same photos on your phone. A gap that wide usually means the three quotes are pricing three different jobs, not that one builder is overcharging. The gap is almost always in a few lines that are easy to skip past, such as allowances, waterproofing, rubbish removal and scope.

My name is Michael. I am a registered builder and a licensed plumber. I have been called out to bathrooms other people built and had to tell the owner the whole room has to come out.

A complete quote and an incomplete one differ line by line. The table below shows what each one includes.

The same bathroom can be quoted either way, and neither builder is breaking a rule. One has priced the whole job and the other has priced part of it.

The first question is scope

Two quotes only become comparable once they describe the same work, and most of them do not.

One builder strips the bathroom back to the studs while another pulls out the fixtures and tiles and leaves the wall sheeting in place. One tiles floor to ceiling on every wall while another tiles the wet walls to the ceiling and the rest to about a metre. One fits a new exhaust fan and ducts it outside while another reconnects the fan that is already up there.

Each of those is a fair choice and none of them is dishonest. They are different jobs at different prices, and a quote that gives you a total with little else cannot tell you which job you are buying.

Allowances are the biggest cause of the gap

Most quotes include allowances. An allowance is a placeholder for something you have not chosen yet, like the vanity or the tiles. It is a guess, not a price, and it gets corrected once you choose the real thing.

Say the quote allows $600 for a vanity. You go to a showroom and choose one you like for $2,400. You pay the extra $1,800, and usually the builder’s margin on top of it. That money was always going to be spent. It just was not in the quote you were comparing.

A low allowance lowers the quote today and raises the bill later.

Victorian contracts have formal names for these. An allowance for something you will choose later is a prime cost item. An allowance for work that cannot be priced until a wall is open is a provisional sum. Both are defined in the Domestic Building Contracts Act 1995, and the Act requires the details to be set out in writing with a breakdown of how the estimate was reached.

The law does put a ceiling on price rises, but allowances sit outside it. In Victoria you can end a major domestic building contract if the price rises by 15 per cent or more, where the increase was something the builder could not reasonably have foreseen when the contract was signed. Increases from prime cost items, provisional sums and variations you asked for are left out of that calculation. An allowance blowout will not give you a way out.

So ask for the allowance schedule before you sign, then take those figures to a showroom and see what they buy. If the vanity allowance is $600 and you have been sending the builder photos of a $2,400 vanity, that gap is already yours.

What gets left out of a quote

Some costs are missing from a cheap quote because they were never priced, not because they were priced low.

Rubbish removal is the first one. A skip and the tip fees run a few hundred dollars, and Melbourne sits at the higher end nationally because of the landfill levy. If it is not in the quote, it comes out of your pocket, usually in the first week.

Permits and compliance are the second. In Victoria a licensed plumber has to issue a Compliance Certificate of Plumbing Work for any plumbing work valued at $750 or more and lodge it within five days. An electrician has to issue a Certificate of Electrical Safety, the waterproofing has to meet AS 3740, and structural changes need a building permit. All of that costs money to produce, so a quote that has not costed it has either buried the cost somewhere else or is not planning to produce it.

Then there is making good, which means plastering the wall where the old vanity sat, painting the ceiling and replacing the architrave that came away with the door. It is the least interesting money in the whole job, and it goes missing from more quotes than any other line.

The last one is site protection and cleaning. Bathrooms usually sit at the back of the house, so every tile and every bag of rubble travels through your hallway. The floors and walls have to be protected for the whole job, and the house has to be cleaned properly before handover.

None of this work is optional and all of it will happen. The only question is whether it was priced before you signed or after.

Waterproofing should be its own line

If one quote is a few thousand dollars cheaper than the others and you cannot see why, waterproofing is the first line to check.

Waterproofing is about 5 per cent of a bathroom, so roughly $1,000 to $1,800 in Melbourne. It disappears from view the moment the tiles go on, which makes it the easiest place in the job to save money without anyone noticing for a year or two. Thinner coats and a membrane that has not fully cured look the same as good work on handover day.

Ask for waterproofing as its own line with its own figure. Ask who applies it and whether that is the same person laying the tiles. Ask what warranty covers it and ask to see that in writing. Ours is a ten year structural warranty. If waterproofing sits inside the tiling number, you have no way of knowing what was allowed for it.

Who is actually doing the work

Two builders can quote the same scope and still land on different numbers because of how their businesses are put together.

Trades are engaged differently from one business to the next. A subcontracted plumber, waterproofer, tiler and electrician all have other jobs running at the same time, so when one of them slips a day the whole sequence has to be rebooked around four other calendars. That is the most common reason a three week bathroom turns into a six week one.

The majority of our trades are employed by us rather than hired job by job. It costs more to run and it shows in the price. What you get for it is a crew that arrives on the day I said they would, and one person to call when something is not right.

It is worth asking any builder who is doing the work, whether they are employed or subcontracted, and what happens to your timeline if one of them does not show up.

The paperwork that protects you

Victorian law puts real protections around a job this size, and most quotes never mention them.

Any domestic building work over $10,000 has to be carried out under a major domestic building contract, and work over that value has to be done by a registered builder. A quote is not a contract, and neither is an invoice.

Work valued at more than $20,000 requires Home Warranty, the scheme that replaced Domestic Building Insurance on 1 July 2026. Your cover starts when you sign the contract. Your builder then pays the premium to the Building and Plumbing Commission within 10 business days of signing, or before work starts, whichever comes first, and you receive a Notice of Cover. Cover runs to $400,000, and you can claim where the work is incomplete, defective or non-compliant and the builder is unable or unwilling to fix it, rather than only where the builder has died, disappeared or become insolvent.

Deposits are capped. On a contract over $20,000 a builder cannot ask for more than 5 per cent up front, so on a $30,000 bathroom that is $1,500. A builder asking for $10,000 before starting is asking for more than the law allows.

You can also check a builder before you hire one. The building regulator publishes a searchable public register of prosecutions and disciplinary action. One builder on it was fined $70,000 for carrying out major domestic building work without insurance while unregistered. It takes about a minute to look someone up, and almost nobody does.

How a cheap quote grows after you sign

Take a job that starts at $22,000. The walls come open and the framing under the old shower has gone soft, so that is a variation. The tile allowance was $40 a square metre and the tiles you chose are $90, so that is another variation with margin on top. The vanity allowance buys a vanity you do not want, so that is a third. Rubbish removal was never in the price. The exhaust fan vents into the roof cavity, and doing it properly costs more again.

By handover the invoice reads $29,000 and the job is three weeks late. Every one of those conversations happened while your bathroom was a hole in the floor, you were showering somewhere else, and saying no was not really an option. That quote was never $22,000. It was $29,000 with the hard parts left until you could not argue about them.

What to do with three quotes

Work through this before you look at a single total.

Check that every line in the table above appears on all three quotes with a figure next to it. Ask each builder for the allowance schedule, then take those numbers to a showroom and see what they buy. Ask what happens if they find rot, asbestos or old pipework, and how that gets priced when they do. Ask whether rubbish removal, making good and the final clean are in the number. Ask for the warranty covering the waterproofing in writing. Ask whether the trades are employed or subcontracted. Ask when your Notice of Cover is coming, and check that the deposit is 5 per cent or less.

If a builder gets uncomfortable during any of that, you have learned something useful and it cost you nothing. The builders who price properly tend to be glad someone finally asked, because it is the only conversation where their number starts to make sense.

Two of your three quotes are probably fine. Choose the one most likely to still be the price at the end, even if it is not the lowest today.

If you want a fixed price with every line itemised, you can book a time with me here. 

FAQ

Why are bathroom renovation quotes so different from each other? Because they usually describe different jobs. Scope, tiling height, allowance levels, whether waterproofing is itemised, and whether rubbish removal and making good are included will account for most of the gap before anyone has overcharged anyone.

What is a prime cost item? An allowance in the contract for a fixture or fitting you have not chosen yet, such as a vanity or tapware. It is defined in the Domestic Building Contracts Act 1995. If your final choice costs more than the allowance, you pay the difference plus the builder’s margin.

What is a provisional sum? An estimate for work the builder cannot price exactly at the time of signing, such as repairs behind a wall that has not been opened. Like a prime cost item, it is corrected against the actual cost once the work is done.

What is a variation? A change to the contract price after signing, either because the scope changed or because an allowance was exceeded. Variations are normal, but a quote built on low allowances makes them close to certain.

How much deposit can a builder ask for in Victoria? A builder can ask for 5 per cent on a contract over $20,000, which is $1,500 on a $30,000 bathroom. Ten per cent applies on contracts under $20,000.

Does my bathroom renovation need Home Warranty cover? Yes, if the work is valued at more than $20,000, which a full bathroom almost always is. Home Warranty replaced Domestic Building Insurance on 1 July 2026. Your cover begins the moment the contract is signed. The builder has 10 business days from that point, or until work starts, to pay the premium to the Building and Plumbing Commission, which then sends you a Notice of Cover. The limit is $400,000.

How do I check a builder in Victoria? The building regulator publishes a searchable public register of prosecutions and disciplinary action. Look the builder up by name before you sign. It takes about a minute.

What should a bathroom renovation quote include? Demolition and rubbish removal, waterproofing as its own line, tiling, plumbing, electrical and fittings, plus the plastering, painting, permits and project management that sit around them. Every one of those should carry a figure next to it.

Should waterproofing be a separate line on a quote? Yes. It is roughly 5 per cent of the job and the most expensive thing in the room to get wrong. If it is buried inside the tiling line you cannot tell what has been allowed for it.

Can I walk away if the price goes up? You can end a major domestic building contract if the price rises by 15 per cent or more, where the builder could not reasonably have foreseen the increase when the contract was signed. Increases from prime cost items, provisional sums and variations you asked for are excluded from that calculation, so an allowance blowout will not give you a way out.

Is the cheapest bathroom quote ever the right one? Sometimes. It is the right one when it describes the same scope as the others, itemises waterproofing, carries realistic allowances and includes the paperwork. If it does all of that and it is still the cheapest, take it.