three months? its usually six months. the trick is to find a decent landlord who cares about the property and is not just bothered about having the rent come in. you should see the house my friends mother has just signed a lease on! she's had to go to environmental health and she's not even in the property yet! lol anyway, regarding your improvements, seek legal advice, most landlords are willing to come to some agreement about lowering rent in light of improvements you make, however its best for you to agree in writing before you make improvements, most landlords also dont just give you notice to move after the initial secure tenancy has passed. they would rather the house was rented than stood empty whilst they find a tenant four times a year! if your looking for a longer security tenancy then i suggest the council or housing associations. you still have rights, he can not just evict you, he can not up your rent with out going through the proper channels, etc. your still protected. presuming your in the uk that is.
edit, just read your reply. i know where your coming from and a decent landlord is hard to find. i had five for just the one house a couple of yrs ago. and never knew where i was, i too wont touch an agency with a barge pole. they cause more problems than landlords realise, its a lazy way of being a landlord. i'm lucky at the moment, my landlady is fantastic. why dont you speak to the landlord about your issues instead of just moaning about it to us? if your good tenants then its extremely unlikely he will want you to leave.