Once granted planning permission normally lasts for between 3 and five years. If the local authority refuses your application it will give you the reasons for their decision and you will then have the opportunity to amend and resubmit your application or appeal the decision.
Planning permission can enhance values before work begins and typically they will also include a number of conditions, for example the scheme must be built in accordance with the approved drawings, trees to be planted as per the landscape scheme and replaced if they die in the first few years, or the colour and finish of external materials to be approved by the Local Authority.