If you are getting involved in rebuilding walls then client must sign the full version of the certificate. Whether the work qualifies for 5.5 will then depend on the exact nature and the percentage of the existing structure which is affected.
http://www.pro-habitat.com/pdf/attestationnormale.pdf
As for your subcontract work the normal rules of VAT apply. You charge the full rate to the main contractor who will reclaim all of the tax you have charged.
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