Ask three builders to price the same extension and you can get three numbers thousands of pounds apart. Usually that's not the builders: it's the drawings. Vague drawings force builders to guess, and builders guess upwards.
1. Groundworks you can't see
Drainage runs, tree roots, and soil conditions are the classic budget-breakers. A measured survey and early drainage check costs little and removes the biggest unknown.
2. Steel and structure
Open-plan spaces need structural steel, and steel needs engineering. Deciding opening sizes early (before planning, not after) keeps the structural solution simple.
3. Glazing
Glass is often the single most expensive line item. A well-placed standard window can do the work of a bespoke sliding wall at a fraction of the price.
4. The specification gap
'Kitchen by client' hides a five-figure decision. A build-ready drawing pack pins down specifications so quotes are comparable and complete.
5. Changes on site
Every change made during the build costs three times what it would have cost on paper. Time spent at design stage is the cheapest money you'll ever spend on your project.
This is the core of what an Architectural Technologist does: resolve the building technically before anyone digs a hole, so the price you're quoted is the price you pay.