What rising build costs and pre-construction delays do to small-site viability

Small development sites rarely fail because of one cost line.

More often, viability gets squeezed by several movements at once: a longer planning period, revised drawings, higher build costs, extra surveys, CIL or Section 106 costs, Biodiversity Net Gain, finance charges, and a contingency that was too thin in the first place.

On a larger scheme, some of that pressure can be spread across more units. On a small site, it can land hard. A three-month delay, a changed specification, or a missed levy can be enough to turn a workable appraisal into a marginal one.

That is why early cost planning matters. A rough square-metre allowance might be useful for a first conversation, but it is a weak basis for buying land, agreeing terms, submitting a planning strategy, or committing to design work.

Planning delay is a cost, not just a wait

The statutory planning period is often described as 8 weeks for smaller applications and 13 weeks for major applications. In practice, the real pre-construction period can be longer.

Applications can be delayed by design revisions, ecology surveys, highways comments, drainage questions, committee dates, legal agreements, pre-commencement conditions, or agreed extensions of time. Even when a decision is recorded as being made “within time”, that can include an agreed extension.

For a small developer, the issue is not just frustration. It is cash exposure.

While the application is unresolved, the site may still be costing money. Professional fees continue. Land or option costs may be tied up. Interest may be running. Build prices can move. The design may change. A planning condition can introduce work that was not in the early budget.

The longer the gap between the first appraisal and a buildable scheme, the more dangerous an old estimate becomes.

Levies and regulatory costs need checking early

Small-site viability can also be affected by costs that sit outside the headline build rate.

Common examples include:

• Community Infrastructure Levy, where it applies
• Section 106 obligations
• affordable housing requirements on qualifying schemes
• highways, drainage, utilities or access works
• ecology surveys and mitigation
• Biodiversity Net Gain requirements
• updated Building Regulations requirements
• energy, ventilation, overheating or fabric specification changes
• planning application fees and consultant input

Some of these costs depend on location. Some depend on site size, use, planning route, floorspace, exemptions, reliefs, or local policy. That makes assumptions risky.

CIL is a good example. Reliefs can exist, including self-build relief in the right circumstances, but they usually come with strict process requirements. Missing a notice or assuming relief applies without checking can change the numbers quickly.

Biodiversity Net Gain is another example. The 10% requirement may affect layout, landscaping, off-site units, legal agreements, or the amount of developable space. It should be treated as a cost and design issue, not paperwork to sort out at the end.

Preliminaries rise when the programme stretches

Preliminaries are often underestimated because they do not feel like “building work” in the same way as foundations, walls or roofs.

But they are real costs.

Preliminaries can include site setup, supervision, welfare, temporary services, scaffolding, security, plant, temporary works, management time, insurance, and other time-related costs. If the construction programme stretches, the preliminaries often stretch with it.

A small site with awkward access, neighbour constraints, phasing issues, party wall matters, limited storage, or restricted working hours may carry higher preliminaries than a cleaner site with the same floor area.

That is one reason generic £/m² allowances can mislead. They may not reflect the way the site has to be built.

Finance and holding costs can eat the margin quietly

Higher interest rates have made delay more expensive.

When borrowing costs were very low, some delays were easier to absorb. That is less true now. Bank Rate rose sharply from late 2021 into 2023, and developer finance, bridging, private lending and refinance costs all need to be viewed more carefully.

The actual cost depends on the funding structure, lender margin, drawdown profile, fees and security. But the principle is simple: every month of delay can add cost before the project has created saleable value.

Holding costs may include:

• land finance
• bridging interest
• loan arrangement or extension fees
• monitoring surveyor costs
• insurance
• council tax or business rates on existing property
• security
• utilities
• professional fees
• abortive design or survey work

On a small site, those costs are spread across fewer units. A cost that looks manageable on a 20-unit scheme may be much harder to absorb on a two-unit conversion or a single infill plot.

Contingency should reflect the risk that is still unresolved

Contingency is not a spare pot for optimism. It is a risk allowance.

The earlier the project stage, the more uncertainty remains. Ground conditions may not be fully known. Drainage may not be resolved. Structural design may still change. Planning conditions may not be discharged. Tender returns may come back higher than expected. The specification may still be moving.

A thin contingency can make an appraisal look better than it really is.

For small sites, it is usually worth testing several scenarios before committing:

• build cost increases by 5% or 10%
• planning takes 3 to 6 months longer than expected
• finance costs rise or the loan runs longer
• CIL, Section 106 or Biodiversity Net Gain costs are higher than first assumed
• abnormal ground, access or drainage costs appear
• sale values soften or sales take longer
• preliminaries increase because the site programme stretches

If the scheme only works in the best-case version, it is not really viable yet. It is just untested.

When to request estimating support

You do not need a full measured estimate for every early idea. But you should get proper estimating support before the numbers start driving decisions.

That usually means before:

• buying or agreeing terms on a site
• submitting a planning application on a marginal scheme
• fixing the design around an assumed budget
• negotiating with a funder or investor
• deciding whether to proceed after planning comments
• comparing contractor prices
• reducing specification to recover margin
• committing to a tender or build programme

A good estimate should do more than produce a single total. It should help you see what is included, what is excluded, what assumptions have been made, where the risks sit, and which cost lines need checking before the next decision.

For small sites, that visibility is often the difference between a viable project and a scheme that only looked viable because the early numbers were too light.