A familiar story: a homeowner and their architect spend months developing a design they love. The planning application goes in, approval comes through, and the drawings go out to tender. Then the prices come back, and they are far higher than the budget allowed.
At that point the options are all uncomfortable. Cut the scheme back and risk losing what made it special. Pay more than planned. Or go back to the drawing board, which can mean a revised application, more fees and months of delay.
The frustrating part is that this is usually avoidable. Most of the cost-defining decisions in a domestic extension or renovation are made long before a builder sees the drawings. Getting practical cost advice before the planning application is submitted, while the design is still flexible, is one of the cheapest and most effective ways to protect a project.
In a recent example, we were invited to tender for the extension and renovation of a Victorian villa. The tender pack was well structured, with detailed construction drawings, notes and engineers’ calculations. We were thanked for our comprehensive proposal, and our costs were confirmed as competitive and in line with the other tenders. But the project was not viable as designed: it came in around 30 per cent over budget.
The pricing was not the problem. The gap was built into the design long before it went out to tender, when a short conversation about cost could have shaped it. Over the following weeks, the scale and specification of the project were reduced to fit the client’s budget, and the architect kept the design they had fallen in love with. A revised application is now with the local planning department.
The extra application and the weeks lost are exactly what an early cost conversation is meant to avoid.
Why the design hardens at planning stage
Once a scheme is approved, it is expensive to change. Depending on the nature of the change, it may need an amendment or a fresh application. Even when it does not, the client has by then mentally committed to the design, and the architect has often moved on to technical drawings built around it.
That makes the period before submission the last point where the building's size, shape, structure and materials can be changed freely. If cost information arrives after that point, it can only react to decisions that are already made. If it arrives before, it can shape them.
What early cost advice is (and isn't)
Early cost advice is not a fixed price. At this stage there is no structural design, no specification and often no ground information, so any number is a range with stated assumptions.
What it should give the client and architect is:
A realistic budget range for the scheme as drawn, with the assumptions behind it spelled out.
The main cost drivers, so everyone knows where the money is going.
Comparisons between options, such as single against two storey, a flat roof against a pitched one, or one large glazed opening against several smaller ones.
A list of risks and unknowns that could move the cost, and what would be needed to resolve them.
The aim is not to arrive at a precise figure. It is to make sure the design and the budget are in the same place before the design becomes hard to change.
The things that quietly drive cost
Clients tend to focus on size, finishes and kitchens. Experience says the biggest swings often come from less visible things:
Structure. Wide openings, steel beams, and the loads they put on existing walls and foundations can change a budget significantly.
Ground conditions and foundations. Trees, clay soils, drainage runs and unknowns below the surface can add cost that is invisible on a drawing.
How new meets old. Tying into an existing building often reveals the condition of what is behind the walls, and opening up early shows what you are dealing with.
Drainage and services. Moving a drain, upgrading an electrical supply or replacing a boiler to cope with the new space can all add to the cost.
Access and logistics. Narrow access, limited storage and the need to keep a family living in the house all affect how efficiently the work can be done.
Specification. The difference between a standard window and a bespoke, large-format glazing system can be thousands of pounds on a single opening.
Heritage constraints. Listed buildings and conservation areas often require specific materials, methods and trades, which affects both cost and programme.
Lead times. Items such as steel, windows and specialist joinery can take weeks to arrive, and delay has a cost of its own.
None of these show up in a planning drawing. All of them can be discussed in a short conversation with someone who builds these projects.
What clients and architects gain
A design that fits the budget from the start. Instead of discovering a gap after tender, the project is shaped to its budget while changes are cheap.
Better decisions, with real information. When a client knows that a particular roof form or opening adds a defined amount, they can choose where to spend and where to save, rather than guess.
Fewer resubmissions and less delay. A scheme that has been sense-checked for cost and buildability is far less likely to need redesign after approval.
A smoother tender. When a contractor has already seen the scheme, the later tender has fewer unknowns, fewer provisional sums and fewer surprises. That makes tenders easier to compare and projects easier to run.
Confidence for the client. Homeowners are about to spend a very large sum on their most valuable asset. Seeing the cost picture early, in plain language, is reassuring and builds trust in the whole design team.
When and how to get it
The best time is during concept design, once the broad layout and massing are understood but before drawings are finalised for submission. A second check as the design develops can be valuable if the scheme changes significantly.
To give useful advice, a contractor needs:
Concept plans and elevations
Any survey information available
The client's budget and priorities
A sense of the quality level expected
In return the client should expect a short, clear summary: a cost range, the main drivers, the key risks and a few options worth considering. For more complex projects, such as period properties or schemes with difficult structure, a more detailed buildability report is often worth the investment.
What it doesn't replace
Early advice does not replace a proper tender. Once the design is developed, with structural information, a specification and a full set of drawings, a formal tender is still essential. But a tender is much more likely to land where everyone expects when it is built on a design that has already been tested against cost.
A few questions worth asking early
Has anyone tested this design against the budget yet?
What are the three biggest cost drivers in this scheme?
What do we not know yet about the existing building and the ground?
Are there simple design changes that would save money without losing the idea?
Are there any long lead-time items we should decide on early?
In summary
Planning permission fixes a design in public. Cost advice, taken before that point, makes sure the design being fixed is one the client can actually build. It takes a modest amount of time at the start of a project and can save a great deal of money, stress and delay later.
If you are an architect or homeowner thinking about an extension or renovation, I would be glad to hear how you approach cost at the early stages, and what you wish had been discussed sooner.
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