S90 refers to a restriction put on the property to the effect that it can only be purchased by a person "in need of housing". Local authorities require purchasers to submit a completed consent form which, depending on the authority contains questions about your current accomodation and your income. Consent is not necessarily confined to first time buyers. The restriction is put on the property when it is first purchased from the council, usually under the tenant purchase scheme. Consent is no longer necessary after the second sale, that is, if you purchase from the tenant purchaser you can then sell to anyone. There is no time limit on your period of occupancy as consent is not necessary when you sell. The selling solicitor usually deals with the consent paperwork and the contracts are usually subject to consent. By the way consent to sale by your vendor is also required, they must satisfy the council that they have somewhere to go and will not seek re-housing from the council.