Two Quotes, One Job: Stop Asking Which Is Cheaper and Ask What Is Inside the Price
The gap between two quotes is usually two legitimate ways of building the same thing — and nobody telling you which one was priced.
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Two quotes for the same job usually differ in 4 places, and none of them are visible when the work is finished: the fixings nobody names, the materials described by category instead of by product, the weeks that were priced out, and the trades that were never listed at all. Wall ties at 2.5 per m², lintel bearings of 150mm, weep holes every 900mm — none of these are expensive, and all of them vanish behind plaster. Labour rates between London contractors vary far less than people expect; scope is what varies. The estimator gives you a line-by-line breakdown to hold both quotes against, and our extensions and loft conversion pages set out what a full scope actually contains.
A developer once put two quotes for the same job in front of me and asked which one was right. So I explained it with a roof — because the difference between those two prices was entirely inside the parts nobody would ever see.
The honest version is not that one builder was cutting corners. It is that the same roof can be built two legitimate ways, and only one of them had been priced.
The two options behind almost every gap
Take one connection in a timber roof. There are two ways to make it, and both are real.
The technical version
A hanger carries its published load only when it is fixed the way the manufacturer tested it — the specified nail, in every hole. That is not a preference; it is the condition attached to the declared capacity, and it is printed on the fixing schedule that comes with the product.
Substitute ordinary round-wire nails for the specified sherardised square-twist ones and industry guidance commonly reports the published capacity dropping by 50 to 75 per cent. Ask to see the fixing schedule. Any builder can produce it in a minute, and it settles the question without either of you having an opinion about it.
This is not good builders against bad builders
Which option you choose is your decision, and your budget and your scope are legitimate inputs to it. If the money has to go somewhere, sometimes it genuinely should go somewhere else in the project. That is a real answer, not a wrong one.
Every builder can do both options. Plenty of builders with years behind them and hundreds of finished projects will give you a beautiful finish either way. The failure is not the cheaper method. The failure is nobody telling you a choice was being made.
For a homeowner building for themselves, I recommend option two — because it is the option I would do for myself, and because in our own book of standards it is what we do by default.
Two roofs, from the top
Imagine two rear extensions, same size, same rooflight, same finish. From the garden in five years they look identical. Inside the build-up, they are not the same object.
On the first, the joist hangers are fixed with the manufacturer’s specified square twist nails, into every hole. On the second, they are fixed with round wire nails from whatever box was open, into about half the holes. A hanger is only rated when it is fixed as tested. Nobody will ever see this.
On the first, the cold roof is ventilated: a continuous 10mm gap at the eaves for a pitch over 15°, 25mm where the pitch is 15° or less, and 5mm at the ridge where the pitch exceeds 35° or the span exceeds 10m. On the second, the insulation was pushed tight to the eaves and the path is blocked. For a year nothing happens. Then the roof void starts running with condensation in February, and the first symptom is a stain on a bedroom ceiling that everybody blames on the tiles.
On the first, every service penetration through a fire-separating element is sealed with a tested fire-stopping system. On the second, the gaps are filled with expanding foam — which is not fire-stopping, has never been fire-stopping, and is the single most common thing I find behind plasterboard.
Neither of those roofs is visibly wrong on handover day. One of them is still quiet in year five.
The figures that get quietly dropped
| The detail | What the standard actually requires | Source |
|---|---|---|
| Wall ties | 900mm horizontal × 450mm vertical, or not less than 2.5 ties/m². Extra ties not more than 300mm apart vertically within 225mm of the edges of all openings. | AD A §2C8 |
| Tie specification | To BS EN 845-1, material reference 1 or 3 (austenitic stainless steel), embedment not less than 50mm in both leaves. | AD A §2C19, Table 5 |
| Lintel bearing | 150mm or greater each end; may reduce to 100mm where the clear span is 1200mm or less. | AD A §2C24(a) |
| Cavity tray and weeps | Weep holes every 900mm. Where the tray stops — above every window and door — stop ends and at least two weep holes. | AD C §5.5(c) |
| Fire-stopping | Every joint, imperfect fit and service opening through a fire-separating element sealed so the fire resistance of the element is not impaired. | AD B Vol 1 §9.1 |
| What is not fire-stopping | Materials must be reinforced with or supported by materials of the appropriate class. Expanding foam alone is not fire-stopping. | AD B Vol 1 §9.25–9.27 |
| Cold roof ventilation | 10mm continuous at eaves over 15° pitch, 25mm at 15° or less, plus 5mm at the ridge over 35° pitch or 10m span. | AD C §6.11 / BS 5250 |
| Extract ventilation | Kitchen 30 l/s with a hood to outside, 60 l/s without. Bathroom 15 l/s. Utility 30 l/s. WC 6 l/s. | AD F Vol 1 Table 1.1 |
Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.
Add up what it costs to do every one of those correctly on a single-storey rear extension and it is a small number. Leave them all out and the quote drops by a noticeable amount, and nothing looks different for two years.
The four places the money hides
1. The fixings and the fittings. Hangers, ties, straps, restraint, padstones, cavity closers, trays, weeps, fire collars. Each is cheap. Together they are a real number, they are all invisible after first fix, and they are the structural and moisture integrity of the building.
2. Materials named by category, not by product. “Rooflight” is a category. A manufacturer, a model and a stated U-value is a product. The gap between the cheapest thing that answers to “rooflight” and the thing you had in mind is often thousands, and the quote is not technically wrong — you simply never said which one.
3. Time itself. Concrete gains strength on its own schedule. Screed dries on its own schedule. Those weeks cannot be bought back, and if they have been priced out of a programme they have not left your project — they have moved into your snagging list. A price can also be lower simply because there are fewer people, or less supervision, on the same number of weeks.
4. The trades nobody listed. Fire-stopping. Cavity trays. Ventilation and commissioning. Making good after the electrician. Scaffold licence. Skip permits. Drain CCTV before you build over a run. Testing. These are not extras. They are the job. But if a quote does not name them, they are not in it, and the conversation about them happens later, from a position where you have already committed.
Reading two quotes side by side
Three moves, in this order, before you look at either total.
- 01Do not start at the total. Find what is in one and missing from the other. The total is the last thing to look at, not the first — it is a summary of two different decisions, and comparing summaries tells you nothing about the decisions.
- 02Give both builders the same list, in the same order. Same items, same sequence, so the prices line up beside each other. If one contractor will not break it down that far, you have learned something already, and you learned it before signing rather than after.
- 03Compare the structural stages against the finishing stages. If structure looks light and finishes look heavy, look again. You may be paying for your kitchen while somebody is still working out how to pour your foundations.
A developer I was sitting with put it back to me better than I had put it to him. He said it is a Mercedes or a Renault — both look like a car, both get you there. Same looking job, but better inside.
Make them describe the same thing first
This is the whole method, and it is simpler than it sounds.
Write one scope and send it to both. It does not have to be a professional specification. It has to be the same words going to both contractors. The moment each builder is pricing their own idea of the job, the two numbers stop being comparable and no amount of staring at them will fix it.
Name products where you care about the product. Rooflights, windows, doors, kitchen, sanitaryware, flooring, ironmongery. Everywhere else, say what performance you need and let them solve it — but know which you have done. In design terms, prescriptive information tells the builder exactly what to install; descriptive information sets a requirement and leaves the answer open. Only prescriptive information makes two tenders genuinely comparable, because only then are both contractors pricing the same object.
Ask for exclusions in writing. Not “is anything excluded?” but “list what is excluded”. A contractor who has priced carefully has this list already and is usually relieved to be asked. A contractor who has not will produce it during the build, one item at a time.
Ask what each assumes about the unknowns. Foundation depth is decided by the ground and confirmed by building control in the open trench, not on the drawing — so ask what depth each quote assumed and what happens if the trench says otherwise. Same with the condition of existing drains, the state of the existing roof structure, and whether there is asbestos. A quote that states its assumptions is not hedging; it is telling you where the risk sits.
Then compare. Once both are describing the same building, a lower price is genuinely a lower price, and you should take it with confidence.
What a good answer sounds like
You are not testing whether a builder is expensive. You are testing whether they have thought about your specific house. Ask what is inside the price and listen for specificity:
- “We’ve allowed 1.0m foundations because of the plane tree three doors down, and if the trench comes up shallower you get the difference back.”
- “That rooflight is the model you sent me. If you want the frameless one it’s a different number and I’ll price it separately.”
- “Fire-stopping and cavity trays are in the price. I’ll photograph both before the boards go on.”
Compare that with “it’s all included, don’t worry”. The second is not necessarily dishonest. It is simply not information, and you cannot buy on it.
The companion question is the payment schedule — where the money sits in time rather than what it buys. Our lesson on front-loaded pricing covers that half.
The takeaway
Do not ask which price is cheaper. Ask what is inside the price — the fixings, the named materials, the invisible trades, the assumptions. Both roofs look the same from the top. Only one of them is still quiet in year five.
The one question to ask both builders
Ask which option is in your price. Both are legitimate — you only need to know which one you are buying.
Then ask both of them about the hangers, and see which one already knew what you meant.
Straight answers
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