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租客已知屋內有加建，租期內租客不可以此作悔約或退租之用.
The tenant is aware of extra area being built in the aforesaid premises, and should not breach or deny the tenancy contract due to this reason.

Tenants are known to have a house to build, rent during the tenant can not make this or regret about tenants terminated.

Tenants are known to have a house to build, rent during the tenant can not make this or regret about tenants terminated.

租客已知屋內有加建，租期內租客不可以此作悔約或退租之用.
The tenant knows about the illegal construction (or additional built areas) of this premises. Within the contracted period, the aforesaid matter shouldn't be a reason for denying the contract.
or you may use "the additional built areas" instead of "the illegal construction".