There are three separate drawing packages behind a normal extension and they are commissioned, priced and read by different people. Confusing them is the most common reason a project stalls: a homeowner with a planning approval and no construction drawings has permission to build something nobody can price, and a builder working from planning drawings alone is guessing at the half of the job that is buried.
Planning drawings prove the form
Existing and proposed plans and elevations, a site plan and a location plan at recognised scales. Their job is to let a planning officer and a neighbour see the size, position and appearance of what you propose. They say nothing useful about insulation, foundations or structure, and they are not meant to. Where the work is permitted development rather than an application, the same drawings are what a lawful development certificate is assessed on, which is why it is worth having them even when no permission is needed.
Building regulations drawings prove the construction
Sections through the walls, roof and floor showing the build up, insulation, damp proofing, ventilation, drainage, fire escape and the structural openings. This is the set building control approves and the set a builder can price line by line. It is normally drawn after planning, because a refusal would waste it, but on permitted development work it can be drawn first and often should be.
Structural drawings prove it stands up
A structural engineer sizes the beams, the padstones and the foundations and issues calculations alongside the drawings. This is a separate appointment, it usually cannot start until the openings are fixed, and building control will ask for the calculations rather than take the beam size on trust. Timber frame work compresses this order, because the frame is designed and often fabricated before anyone is on site, so the structural and construction information has to be complete earlier than a masonry job would need it.
Rear extension floor plans are a use test, not a drawing exercise
A floor plan is worth its fee when it has been tested against furniture, doors that swing, a route to the garden and a place for the things a household actually owns. On a terrace the plot does most of the deciding: the width is fixed, the light comes from two ends, and the only real variables are how deep you go and where the opening between old and new sits. Draw the furniture in before the walls are agreed.
Questions people ask about house extension plans
Can one set of home extension drawings do all three jobs?
No. They are read by a planning officer, a building control officer and a structural engineer, and each needs information the others do not show.
Do I need plans if the work is permitted development?
You still need construction drawings to build and price it, and drawings are what a lawful development certificate is judged on if you want the permission in writing.
Which comes first in timber frame extension plans?
More of the detail comes earlier, because the frame is designed and made off site. Late changes are much more expensive than on a masonry build.
What scale should extension plans be drawn at?
Use the scales your local authority's validation list asks for. Submitting at the wrong scale is a common and entirely avoidable reason for a submission being returned.