There's a real difference between an estimate built from a conversation about the job and a construction estimate from drawings. One is a memory of what someone said. The other is a measured record of what's actually been designed. Only one of them survives contact with the site.
Most builders know this. Most builders have also priced a job off a walk-round and a chat, because the drawings weren't ready or there wasn't time to read them properly. That's where the trouble tends to start.
The drawings are where the real quantities live
A rough idea of a job misses the details that only show up when you read the drawings properly: changes in level, junctions between trades, and specification notes buried in a corner of the sheet in small type.
Take a simple example. A client describes a kitchen extension as "about five metres by four, flat roof, bifolds on the back." Sounds straightforward. The drawings then show the floor dropping a step to meet the garden, a raised lantern on the flat roof, a new soil stack routed through the wall, and a note saying the foundations are to be designed by the engineer once the trial holes are dug. None of that came up in conversation. All of it costs money.
Reading the drawings properly is a skill of its own. Check the scale bar rather than trusting the title block, especially on a PDF that's been exported or printed at a different sheet size. Check figured dimensions against scaled ones. Cross-check plans against sections and elevations, because a wall height on the section that doesn't match the elevation is a question to ask before you price, not after.
That's not the client being difficult. People describe what they care about, and most homeowners care about the bifolds, not the drainage.
What a construction estimate from drawings catches
Two people describing the same job can leave out different things. Measuring the actual drawings to NRM2 means the estimate is built from what's specified, not from what was remembered. In practice, that tends to pick up:
- Level changes. Steps, ramps, thresholds and the extra excavation or build-up they need.
- Junctions. Where the new roof meets the old wall, where the render stops and the brick starts, where one trade hands over to the next.
- Specification notes. A particular insulation board, a fire-rated door, a specified brick to match existing. Small print, real cost.
- Things drawn but not discussed. Soakaways, lintels, cavity trays, trickle vents on the window schedule.
- Revision differences. What changed between the version you saw last month and the one issued for tender.
Individually, none of those is dramatic. Together, they're the gap between a price you can build to and one you'll be topping up out of your own pocket.
A conversation tells you what the client wants. The drawings tell you what you'll have to build. Price the second one.
The estimate holds up because the source does
When someone asks how a figure was reached, "measured off the drawings you provided" is an answer that holds. "Priced off what we discussed" doesn't, once the job's under way and the drawings say something different.
This matters most when things change. If your estimate is tied to a specific drawing revision, you have a clear baseline. The client moves a window, the architect adds a roof light, and you can show exactly what's different and price just that. Without that baseline, every change becomes a negotiation about what was "included", and you're arguing from memory against someone who has a set of drawings in front of them.
It matters for your subcontractors too. If you're getting prices in for the roofing, the electrics or the groundworks, they'll price from the drawings. If your main estimate was built from a conversation, the numbers won't line up and you won't know why until the invoices arrive.
Working from what you've actually got
Not every job arrives with a complete drawing pack. Sometimes it's planning drawings only. Sometimes it's a PDF of a hand sketch with a few dimensions. That's fine, as long as the estimate is honest about it.
A good estimate built from partial drawings measures everything that is shown, then lists clearly what isn't: "foundations assumed as strip footings to one metre depth pending engineer's design", or "kitchen supplied by client, fitting only included". Those assumptions are part of the estimate, not an apology for it. They tell the client exactly where the price could move and why.
The point is that a construction estimate from drawings is still the right approach when the drawings are thin. You measure what's there and fence off what isn't, rather than letting a guess quietly fill the gap and hoping nobody notices until the final account.
At MPE we measure every element off the drawings you send, to NRM2, and price it on current UK rates local to the job. Where the drawings leave a gap, we say so in writing. You can see how that's laid out in our construction estimating services, or look at a sample before you send anything.
Is your price built on paper or on memory?
Here's the test. Pick any line in your estimate and ask where the quantity came from. If you can point to a drawing, a dimension and a revision letter, that line is solid. If the honest answer is "that's about what these usually take", you've found a line that was priced from a rough idea.
One or two of those might be fine. A whole estimate of them is a guess with a letterhead on it, and the site will find it out long before the client does.