As the majority of garage conversions are considered permitted developments or pd, it is not necessary to request planning consents in Hackney. Within your house deeds there could be a restrictive covenant which requires that the garage be kept as a parking area, these will need to be discharged. In Hackney you would also require formal approval to alter the use of a detached garage.
It is possible that your pd rights may be revoked, however it is still possible for you to ask for approval from householder planning by putting together any design sketches and approaching them for the requisite permissions. The overall charge would be £500 and you would have to pay the design fees on top of that. Large structural alterations such as changing windows, or incorporating an extension would require you to complete an application. Depending upon the property where you wish to carry out the conversion, we may need to apply for various permits, for example if you reside in a listed building, or you have adjacent neighbours.