Where an addition meets original fabric there is almost always a line. These signs place that line. What they establish is sequence, not legality.
1
A vertical crack running floor to roof
Original fabric and a later addition move independently and separate at the junction. The crack runs the full height rather than radiating from an opening.
Exterior wall, at a change of plane
2
Render texture or wall plane changing
A step in the wall face, or a visible change in render grain and colour that follows a straight vertical line rather than weathering gradually.
Facade and rear elevation
3
Roof pitch or tile batch changing
Tiles fired decades apart differ in tone and profile. A shallow section joined to a steeper original, or a change of tile across one plane, marks an extension.
Visible from an upper floor opposite
4
Ceiling height changing at a threshold
A step down into a room, or a drop in ceiling height crossing a doorway with no structural reason on the drawings, usually marks where the building stopped and the addition began.
Interior, at room junctions
5
Window rhythm broken
Original elevations follow a repeating rhythm of opening size and spacing. A window that sits outside that rhythm, or one with a different reveal depth, was cut later.
Whole elevation, from across the street
6
Floor finish changing with no doorway
A change of flooring material or level at a line where no door exists is a boundary between two construction phases that has been finished over.
Interior, ground and top floors first
The distinction that matters
Every sign here identifies later work. None of them identifies unauthorised work. Extensions carrying full municipal authorisation look identical from the pavement, and plenty of them are. What the signs do is tell a buyer which part of the building to ask about, so the question can be put against the approved drawings and the municipal file rather than guessed at.