VQOTW: PROPERTIES ON HAND AT DEREGISTRATION

One of my clients deregistered recently and has now received a letter from HMRC asking about a property that he bought four years ago for £150,000 and opted to tax. Although there was no VAT chargeable on the purchase, the property was empty at the time and in quite a poor condition as the seller was reluctant to spend money on it, so my client also spent £100,000 doing it up ready to let out. He opted to tax so that he would be able to recover the VAT on this refurbishment, and until deregistering has charged VAT on the rent. What does he need to do? As he has already cancelled his registration is he going to receive a huge assessment? The property is now worth around £300,000.