Level 2 and level 3 differ in three ways: how far the surveyor may go in investigating, how much they must explain, and how much professional risk they carry. Everything else people cite as a difference, length, photographs, price, follows from those. The choice is decided by the building's age, its alterations and whether anything is already visibly wrong.
What changes on site
Level 2 is a visual inspection of what is accessible. Level 3 extends to what can be reached safely with a ladder and a hatch, and to opening what can be opened without damage. On a house with a suspended floor and an accessible cellar that is a real difference.
What changes in the report
Level 2 rates and comments. Level 3 must explain the cause of each defect, say how urgent it is, and recommend a remedy. If your worry is why something is happening, level 2 will not answer it. That obligation to explain is also what makes the top level report usable when you go back to the seller with a number.
Where the money goes
Time on site, time writing and professional risk. A level 3 report that explains a cause commits the surveyor to a view, and that is priced. It is also what makes the report usable in a price negotiation.
A simple decision rule
Conventional, unaltered, apparently sound, built in the last few decades: level 2. Old, altered, extended, unusual construction, flat roofs, cellars, or visible movement: level 3. When it is genuinely borderline, describe the house to two firms and take the advice, which is free.
Questions people ask about level 2 or level 3 survey
Is level 3 always better?
It is always more detailed and always more expensive. On a straightforward modern house the extra depth buys very little, so better depends on the building.
Can I get a level 2 and upgrade if something turns up?
Some firms will revisit, most will prefer to have booked the right level. Ask before the inspection rather than after, because the surveyor plans the visit around the level.
Does the difference between level 2 and 3 matter to a lender?
No. Lenders rely on their own valuation. The level matters to you, not to them.