legal alterations to a tenancy agreement

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We have a tenanacy agreement which is standard in the way it is written , what i have been wondering is , can we put extra conditions in the agreement i.e you must mow the lawn at least four times a year , and on vacating the property(when the tenancy ends or evicted)all personal belongings on the property must be removed prior to your tenancy ending,failure to comply will result in anything being left on the property will be skiped or towed away with no compensation or recourse to the landlord .
Or something along those lines anyway any suggestions would be helpfull on what is legal towards alteration of a tnancy agreement.

Thanks T & D