As the majority of garage conversions are considered permitted developments or pd, it is not necessary to request planning consents in Westminster. 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. By changing the original use of your detached garage, you may be required in Westminster to apply for formal approval.
Even if pd rights have been revoked, you will still be able to follow a scheme however you would need to put together appropriate sketches and ask for approval from householder planning. The cost would be £500 with any additional design fees as extra. Large structural alterations such as changing windows, or incorporating an extension would require you to complete an application. Our methodical team can help you with any other permits that you may require such as listed building consents or party wall agreements.