CIL charging - Public forum - Planning Advisory Service (PAS)

We have a CIL threshold of £75 for 1-10 resi units and £150 for 11 +units. If a developer submits an application for 15 units, 6 of which are affordable and therefore not liable for CIL what should you base the CIL liability calculation on- 9 x £150 or 9 x £75?