The construction of a new attached carport, open on at least 2 sides, would not normally require Building Regulations if it is less than 30m2 in floor area. Construction of a non combustible detached garage less then 30m2 would not normally require Building Regulations approval.
Construction of a small detached building such as a garden shed or summerhouse in the garden will not normally require Building Regulations if the floor area of the building is less than 15m2. If floor area is between 15 & 30m2 you will not normally require Building Regulations providing that the building is either at least 1m from the boundary or it is constructed of substantially non-combustible material.
Windows & doors
If you want to replace broken, fogged double glazing units, rotten sashes or rotten sections of the main frame members, Building Regulations will not apply, providing they are the same size and installed by a 'competent' installer, see the section on Competent Persons Schemes for an explanation.