Planning permission feels like an ending. Drawings approved, council satisfied, time to find a builder. That feeling is the problem, because at the point permission arrives the design that gets built has barely been started.
Most extension budgets are lost in the gap between two things that look similar and are not: a planning drawing and a construction package. Understanding what sits in that gap explains most disputes an architect in london sees on domestic projects.
Planning Approval Sits at the End of Stage Three
The RIBA Plan of Work 2020 divides a project into eight stages. Planning applications are usually submitted during or at the end of Stage 3, Spatial Coordination.
Stage 3 is not the design stage most people assume it is. Its job is to test and validate the concept agreed at Stage 2 and to coordinate architecture with structural and services engineering, so that nothing collides later.
Stage 4, Technical Design, is the one that produces detailed drawings and specifications capable of being built from, along with the Building Regulations submission. It is also, explicitly, the stage that forms the basis for tender packages and contractor procurement.
So planning consent marks the end of coordination, not the end of design. There is an entire stage of work between approval and a builder being able to price the job properly.
The RIBA Says This Directly
This is not an opinion from the profession’s fringes. The RIBA’s own overview of the Plan of Work addresses the practice of tendering on Stage 3 information and warns against it.
Its position is that while Stage 3 information is coordinated, it is unlikely to contain enough detail for robust tenders to be prepared, because specifications will not be ready, the scope of work may be incomplete, and detailed drawings will not yet have been produced.
Read that as a description of what a builder receives when a homeowner hands over a set of planning drawings and asks for a price. Three things are missing, and each one becomes a cost later.
What Actually Gets Added Between the Two
| Planning stage information | Technical design information |
| Plans, elevations and sections at planning scale | Construction drawings with junctions and details resolved |
| Massing, form and appearance | Written specification of materials and workmanship |
| Design and access statement | Structural calculations and steel schedules |
| Site plan and location plan | Building Regulations submission package |
| Indicative window and door positions | Confirmed products, sizes and performance data |
| Broad indication of materials | Drainage layout, falls and connection details |
| Nothing about buildability | Specialist subcontractor design, allocated to Stage 4 |
That right hand column is what a builder needs to price accurately. Without it, the builder prices assumptions, and every assumption is a variation waiting to happen.
How the Money Actually Leaks
Three mechanisms, all of them predictable.
Provisional sums. Where the specification is absent, a builder inserts an allowance rather than a price. Those allowances are placeholders, and the final figure is whatever the item costs once someone finally chooses it.
Variations priced without competition. A change requested after contract is priced by the only party in a position to price it. On a competitively tendered job you have a baseline to test that against. On a job tendered from incomplete information, half the project becomes variations.
Site queries. Poor information at technical design stage generates queries during construction, and each one is a pause. Outcomes compound through the stages, which is precisely why the Plan of Work exists as a sequence rather than a menu.
Nobody Is Watching the Handover
The second failure is structural rather than informational. On many domestic projects the architect’s appointment ends at planning, and nobody is formally administering the contract during the build.
That matters because somebody has to decide whether work matches the drawings, whether a variation is justified, and whether an invoice reflects work actually completed. Without a contract administrator, those judgements fall to the homeowner, who is negotiating with a builder about a technical standard they have no way to assess.
It also leaves the clash question unowned. When structure conflicts with drainage, or a services route will not fit the ceiling void, someone has to resolve it. On a fragmented appointment that person is frequently whoever is on site that morning.
Where Design and Build Changes the Picture
Consolidating design and construction under one contract removes the handover as an event, because there is nothing to hand over. The party producing the technical information is the party building from it.
A design and build arrangement therefore transfers the coordination risk rather than leaving it with the homeowner. Where structure clashes with services, that is a contractor problem to solve rather than an argument between two firms with separate insurance.
What it does not do is remove the need for technical information. It moves who produces it. The Plan of Work is explicitly usable under design and build, and Stage 4 still has to happen. A contractor pricing from Stage 3 drawings under a fixed price is exposed to the same gaps, which is why the requirements document matters so much on that route.
The trade off is design control. Once the contract is signed, detailed decisions sit with the contractor, so the specification needs to be genuinely fixed beforehand rather than left to discretion.
What to Ask For Before Tendering
The diagnosis suggests a short list of questions, and the answers separate a complete appointment from a partial one.
Ask which RIBA stages the fee covers, and whether Stage 4 is included or quoted separately. Ask whether structural engineering sits inside or outside the appointment. Ask who submits the Building Regulations package, and when.
Then ask who administers the contract during construction, who values the work, and who decides whether a variation is legitimate. If the answer to all three is nobody, the project has a gap in it regardless of how good the drawings are.
Finally, ask what happens to the design if the builder proposes a change. Whether the answer is a novation, a retained architect or a contractor’s own team, it should be settled before anyone signs, because retrofitting the arrangement mid build is considerably harder than agreeing it at the outset.
None of this makes the planning drawings wasted work. It just means they are the halfway point rather than the finish line, and treating them as the latter is what turns a well designed extension into an expensive one.
