The other test is sound. Tap the render with a knuckle or a coin — a hollow note means it has lost its bond to the wall and will come off in a sheet whatever you fill the crack with.
Reading the crack pattern
Vertical cracks at the junction of an extension and the original house are usually differential movement, and are best handled with a proper movement joint rather than repeated filling.
Stepped cracks following the mortar joints below the render suggest the masonry is moving, not the coating. Horizontal cracking at floor level in a rendered timber-frame or a rendered blockwork wall often points to the substrate rather than the finish.
Patch repairs that actually hold
A sound patch means cutting back to well-bonded render with a clean, undercut edge, dubbing out in stages rather than one thick coat, and matching both the mix and the surface texture. Feathering thin edges over sound render is what makes patches show and fail.
Colour match is the honest limitation. On a painted elevation the patch disappears once the wall is coated; on a through-coloured silicone or a bare lime render, a patch will always be visible to some degree.
When to strip the whole elevation
Once around a quarter of an elevation sounds hollow, patching becomes a rolling cost. Stripping back, repairing the substrate, and re-rendering with a system suited to the wall — lime on soft solid masonry, silicone or mineral on modern block — gives you one job and one guarantee instead of five visits.
Scaffold is usually the biggest single line in the price, which is why it pays to do adjacent work such as fascias, guttering and repointing at the same time.
Common questions
Are hairline cracks in render serious?
Usually not on their own. They matter when they let water in behind the render, so they are worth sealing and overcoating even when they are cosmetic.
Why does render sound hollow?
It has debonded from the wall behind, often through a weak or dusty substrate, a suction problem when it was applied, or water tracking behind it.
Can you render over old render?
Only where the existing coat is fully sound and compatible. Over a failing or non-breathable coat, it just adds weight to something already letting go.
Lime or cement render?
Lime on soft, solid, older masonry so the wall can dry. Cement or modern silicone systems on dense modern blockwork designed for them.