The engineer needs to understand the existing structure and the loads proposed above or alongside it. [1] [2]
First identify what the project actually is
Distinguish between converting the garage, extending beside it, replacing it with a larger structure and adding an upper storey. These proposals can have different implications, even when they appear together on the same sketch.
Mark whether the garage formed part of the original house or was added later. Under the householder rules, original means the house on 1 July 1948, or as first built if later. The relationship to the original side and rear walls affects the permitted development assessment. [2]
Do not assume the existing foundations are adequate
An engineer may require trial pits or other investigations to establish the foundation form, ground conditions and existing construction. A garage that has remained standing under its present loads has not thereby demonstrated capacity for an additional floor. [1]
Even a garage conversion can require assessment of the foundation beneath a new infill wall at the former door opening. Planning Portal's guidance identifies existing foundation suitability, soil, trees and nearby drains as relevant matters. [3]
For an upper-floor addition, ask the structural design to explain the complete load path. If new supports are proposed inside or beside the garage, show their effect on the space and access below before assuming that parking or storage can remain unchanged.
Check Class A and Class AA carefully
Under Class A, an extension projecting beyond an original side wall cannot have more than one storey. That limitation can prevent a proposal over a side garage from qualifying, even where the extra room is modest. [2]
The separate upward-extension right, Class AA, is not a general right to build over every lower part of a house. Planning Portal explains that the principal part excludes lower front, side or rear extensions. Do not assume that a lower side garage forms the qualifying principal part. [4]
Where permitted development is unavailable, consider a planning application using the council's current design guidance. Show the relationship of the upper-floor addition to the main roof, adjoining house and street. [5]
Coordinate the old and new construction
The proposal needs an appropriate building regulations design for structure, fire safety, thermal performance and ventilation. Where the garage is retained below accommodation, its relationship to the dwelling must be considered in the fire-safety design. [6] [7]
Ask for sections through the floor, garage and roof junctions, not just a new bedroom plan. Confirm the investigation findings before committing to a construction price that assumes the existing garage will carry everything.
Nearby excavation or work to a shared wall can trigger party wall procedures. These are separate from the planning and structural assessments, including where the garage appears to sit wholly within your property. [8]