Normally if you are not able to release equity in the final year of your IVA then creditors would expect you to extend your IVA by a further twelve months in liue of this equity. However if there is no equity there based on 85% LTV then there would be no reason for an extension.
So if your IVA was proposed over 60 months they it could be extended to 72 months.
If your IVA's original term was 72 months then yes it could technically be extended by a further twelve months, however as you have already done a 6 year term i suspect creditors would be happy with this.