You will need to obtain accurate dimensions for the converted garage and rest of the buildings, perhaps from the local council. It is not a requirement that building plans be professionally drawn up, you can draw up the plan yourself, provided the measurements are accurate.
You will not qualify for a deduction for the improvements made when converting your garage to a home office, as this does not qualify as a repair. You may, however, qualify for a wear-and-tear allowance for the chair and desk.
Further, expenditure and losses incurred prior to the commencement of trade would not be eligible for deduction as these are not permitted under the legislation applicable to home office claims.