I'm confused here...
We're deducted 20% when requested for CIS number on some jobs. Should I not be allowing it?
If the builder is part of the CIS and you have no exemption then you have no choice. Just be sure to get your monthly CIS statement.
Your remittances will also clearly show the deductions so worst case you'll have that to fall back on should there be any issues.
All works we carry out for all builders, including sales centres and show homes automatically attract the 20% deduction.