Once a development has been approved it must be carried out in accordance with the approved plans and any conditions attached to the permission. Your planning permission may include conditions requiring you to submit further details for approval of the Council within an agreed timescale, often before development commences. It may be necessary to re-consult on these further details.
The case officer will normally be able to advise as to whether any changes need to be formally approved by the Council, and it is recommended that written confirmation obtained of any amendments is sought. If further plans are required it should be made clear on plans what changes are being made to the original permission. If the changes are deemed to be different from the original proposal it may be necessary to submit a new application. The responsibility lies with the applicant or the developer to notify the Council if amendments to the approved plans are necessary. It is also worth remembering that just because planning permission has been granted you may also have to comply with other requirements like the Building Regulations.
You may also need to get your landlord's approval if the property is leasehold, and there may also be restrictive covenants, which affect your property. If you are unsure about this, you are advised to take legal advice before commencing work.