The firm said garden centre concession tenants are also are trading after an agreement has expired and have not reviewed their tenants agreement terms since they were installed.
First Franchise business development manager Richard Lewis said: "All of these can leave garden centres exposed to legal ramifications if not resolved, and not having the right paperwork will affect the overall value of a site.
"Thus as an owner of a garden centre thinking of selling or even planning for the future, having a secure concession agreement is integral to the business model.
He added: "In addition, in some cases agreements are rolled over with no review of rent and terms so, tenants on site not paying the market value will also have a negative impact on the total valuation potential. Tenants also need secure agreements. Therefore, garden centres should not be concerned about disrupting the status quo."