Schemes proposing 6 units (35.7% refusal) and 9 units (34.3% refusal) have materially lower refusal rates than their neighbours. The 9-unit dip reflects affordable housing threshold optimisation. The 6-unit dip, less discussed, may reflect a massing sweet spot for typical small sites.
Are there unit count sweet spots in London planning?
Developers talk about sweet spots — the unit counts that seem to get through planning more easily. Is this received wisdom, or does the data bear it out? We looked at refusal rates by proposed unit count across 11,989 London small site applications to find out where the inflection points actually sit.
Two dips at 6 and 9 units — different mechanisms, same result
The refusal rate for small sites in London is not a smooth curve. It has two distinct dips.
At 6 units, the refusal rate drops to 35.7% (n=342). At 9 units, it falls to 34.3% (n=297). Everything around them is higher.
| Units Proposed | Refusal Rate | Applications |
|---|---|---|
| 1 | 44.7% | 5,918 |
| 2 | 41.7% | 2,750 |
| 3 | 45.5% | 1,062 |
| 4 | 44.8% | 654 |
| 5 | 44.4% | 441 |
| 6 | 35.7% | 342 |
| 7 | 43.4% | 252 |
| 8 | 45.4% | 173 |
| 9 | 34.3% | 297 |
The plateau from 1 to 5 units hovers between 41% and 45%. At 6 it drops by 9 percentage points. At 7 it bounces back to 43.4%. At 8 it peaks at 45.4%. Then at 9 it drops again to the lowest rate of any unit count in the dataset.
Two valleys. Two different explanations.
The 9-unit dip is the one most developers already understand. Ten units triggers affordable housing obligations in most London boroughs — typically 35% to 50% of units, or a substantial financial contribution. A scheme of 9 units sits just below that threshold. Developers proposing 9 units are, by definition, optimising. They have designed to a specific constraint.
But this creates a selection effect that is important to acknowledge. The 9-unit cohort is not a random sample. These are schemes where someone — usually an experienced developer or their planning consultant — has deliberately calibrated the proposal. The schemes are likely to be better designed, better supported with technical assessments, and submitted to boroughs where the developer already understands the planning environment. The unit count does not cause the approval. The sophistication behind choosing 9 units probably does.
The 6-unit dip is less discussed and harder to explain through self-selection alone.
Six units sits below the committee referral threshold in most boroughs. Schemes of 7 or 8 units are more likely to go to planning committee rather than be decided under delegated authority. Committee decisions on small sites correlate with higher refusal rates across the dataset — members are more risk-averse than officers on schemes at this scale. A 6-unit scheme is large enough to generate meaningful GDV but small enough to stay on an officer’s desk.
There may also be a massing explanation. On a typical London plot — 150 to 250 square metres — six units often represents the natural capacity of a two-and-a-half or three storey building with sensible room sizes. It is the point where the architecture does not have to fight the site. Seven or eight units on the same footprint requires either an additional storey or tighter internal layouts, both of which attract objections.
The 8-unit peak at 45.4% is notable. It combines the worst of both effects: large enough to trigger committee in many boroughs, not large enough to justify the design investment of a 9-unit optimised scheme. It is the unit count that falls between two stools.
How to use the 6-unit and 9-unit sweet spots
Consider a developer working up a scheme on a 200 square metre site in south London. The massing study suggests the site can accommodate 7 to 9 units. How should they choose?
A 7-unit scheme has a London-wide refusal rate of 43.4%. It may go to committee, depending on the borough. It sits in the dead zone — above the 6-unit sweet spot, below the 9-unit one.
A 9-unit scheme has a 34.3% refusal rate. It stays below the affordable housing threshold. In most boroughs, it remains within delegated authority. The additional two units add roughly £800,000 to £900,000 of GDV (at typical outer London values), and the statistical probability of approval is 9 percentage points better.
The trade-off is design quality. Fitting 9 units where 7 sit comfortably demands more from the architecture — better section design, more creative amenity provision, careful daylight geometry. The 9-unit scheme that achieves a 34.3% refusal rate is not the one where two extra flats are squeezed into the roof space. It is the one where 9 units were designed from the outset as the optimal capacity of the site.
This is where the self-selection effect works in the developer’s favour, if they are aware of it. The 9-unit cohort performs well because it is composed of well-prepared applications. Join that cohort. Design to 9 with the same rigour, and you inherit the statistical advantage.
The 6-unit logic operates differently. If the site naturally accommodates 6 units without strain, do not push to 7 for the additional revenue. The jump from 6 to 7 adds one unit of GDV but nearly 8 percentage points of refusal risk. On a risk-adjusted basis, the seventh unit may cost more than it earns.
Why these are correlations, not guarantees
These are London-wide figures. Individual boroughs show different patterns — the 6-unit and 9-unit dips are not uniform everywhere. Sample sizes at specific unit counts are modest (342 and 297 respectively), so borough-level breakdowns become thin. The analysis also cannot control for site size, location, or design quality. A 9-unit scheme on a large site and a 9-unit scheme on a cramped one are counted identically. And the self-selection caveat on the 9-unit finding is genuine — this is an observed correlation, not a causal claim.
Borough-level unit count profiles, committee thresholds, and affordable housing cliff effects are detailed in each of the all 33 borough reports.