Planning required if it doesn’t fall within permitted planning. Have a look at the planning portal website for more info. If you can use a ruler and a scale rule I’d do this myself if I was you- it dosent need to be anything fancy.
Building Regs will be required but I’d do this via a ‘building notice’ rather than submit plans.
If you’d rather have someone else sort all that you could either use an architectural technician or a builder who can get them drawn up.
For the roof you can either use pre-fabricated trusses to form the pitch or have a carpenter construct a ‘cut roof’. The cheapest may depend on the size of garage you have. The roof will have to be strapped down to the existing structure and lead flashings installed at any abutments, but any decent builder will do this.
For such a simple job I wouldn’t expect the builders would need any drawings to work from unless you have any specific requirements, but by the sounds of it you don’t. Just make sure you have at least three quotes and that the work includes everything.