The short answer
With full plans, building control checks your drawings and approves them before you build. With a building notice, you tell them you are starting and they check the work on site as it goes. Fees are similar. For an extension built from a kit, full plans is the better route, because the drawings and calculations exist anyway and the kit cannot be altered once it is made.
The two routes in one paragraph each
Full plans. You submit drawings, a specification and structural calculations to building control with a fee. They check them against the regulations and reply within five weeks, or two months if you agree to an extension. The reply is an approval, an approval with conditions, or a rejection with reasons. Once approved, you build to the drawings and the inspector visits at set stages. If the work matches the approved plans, you have a strong answer to any later argument about compliance.
Building notice. You send a short form, a site plan and a fee, and can start two days later. Nothing is approved in advance. The inspector looks at the work on site and can ask for it to be changed or opened up if it does not comply. They can also ask for drawings or calculations at any point, so “no plans” often turns into “plans, but later”.
Both routes end the same way, with a completion certificate if the work passes. Both lapse if work has not started within three years.
When a building notice cannot be used
The notice route is not available for every job. You must use full plans if the extension will be built over or close to a public sewer shown on the water company’s map, and if the building is one where fire safety law applies, such as a shop or a house converted to flats. For an ordinary house, the sewer rule is the one that bites: on many terraced streets the shared drain runs along the back of the houses, exactly where the extension goes.
What each route costs you
Council fees for the two routes are usually the same or close to it. The difference is where the risk sits.
On a building notice, a mistake is found after it has been built. If the inspector decides your footings are too shallow, your insulation too thin or your beam undersized, you pay to put it right. Builders who do the same kind of job every month and know their local inspector use the notice route happily, because they already know what will be accepted.
On full plans, a mistake is found on paper. Changing a drawing is free.
Why full plans suits a kit
Three reasons make the choice simple for anyone buying a kit.
The paperwork already exists. A supplier of extension kits produces frame drawings and structural calculations as part of the order. Add a floor plan, a section, a foundation detail and an insulation specification and you have a full plans submission. The saving in effort from a building notice is small.
The kit cannot be changed afterwards. Panels are made to the drawing. If building control wants a thicker wall or a different lintel after the kit is delivered, you are altering factory-made parts on site. Get the approval first, then place the order.
You are probably not a builder. The notice route relies on the person building knowing the regulations well enough to get it right unprompted. An approved set of drawings gives a first-time builder something to follow and something to show each trade.
Council or private building control
You can use the council’s building control team or a private registered building control approver. Private approvers do not use the terms full plans and building notice. They file an initial notice with the council and then check your drawings and inspect in much the same way. Ask a private approver whether they will review the plans before you order the kit. Most do.
Scotland
None of this applies in Scotland. There is no building notice. You apply to the council for a building warrant with drawings, and it is an offence to start work before the warrant is granted. At the end you submit a completion certificate for the council to accept. Allow for the warrant in your programme before you fix a delivery date for a kit.
If you have already started on a notice
You can still protect yourself. Send the inspector the frame drawings and calculations before the kit is ordered and ask, in an email, whether they have any comments. It is not a formal approval, but it surfaces most problems while they are still cheap.