The Regulation 40 Mathematical Formula
Under Regulation 40 of the CIL Regulations 2010 (as amended by the 2019 Regulations), the collecting authority must calculate the levy liability using the formula:
Liability = (R × A × Ip) ÷ Ic
Where:
• R is the charging authority's base rate (£/m²) for that specific use class and geographical zone.
• A is the net additional chargeable area (m²) measured in Gross Internal Area under RICS Code.
• Ip is the index figure for the calendar year in which planning permission was granted.
• Ic is the index figure for the calendar year in which the charging schedule took effect.
| Calendar Year | RICS CIL Index Figure | Relative Inflation vs 2015 Base | Impact on £100/m² Base Rate |
|---|---|---|---|
| 2015 | 258 | Baseline (1.00x) | £100.00 / m² |
| 2018 | 313 | +21.3% (1.21x) | £121.32 / m² |
| 2021 | 332 | +28.7% (1.29x) | £128.68 / m² |
| 2024 | 381 | +47.7% (1.48x) | £147.67 / m² |
| 2026 | 415 (est.) | +60.8% (1.61x) | £160.85 / m² |
Deducting Existing Buildings: The 6-in-36 Month Lawful Use Rule
Developers can dramatically reduce their CIL bill by claiming existing in-use building deductions under Regulation 40(7).
To qualify, the existing building must be a permanent structure and must have been in continuous lawful commercial or residential use for an unbroken period of at least 6 months within the 3 years (36 months) ending on the day planning permission was granted.