Everything else on this site is an estimate built from numbers you typed. A survey is the only thing that can see your roof, and these are the five things it looks at, in the order they change the price.
Under every tile is a batten, and under that is the felt. Battens that have carried a heavy concrete tile for forty years are often sound and sometimes are not, and a batten that will not take a fixing has to be replaced along its whole run rather than in the one place it failed. This is the single largest thing a quote cannot know in advance.
Felt fails at the eaves first, where it is wettest and where it is folded. If it has gone, the roof has been shedding water into the top of the wall for some time and the timber below the eaves needs looking at before anything goes back on.
Hips, valleys, dormers, chimneys and any change of pitch are all cuts, and every cut is labour and waste. The complexity uplift in the estimator is one number standing in for a count, and a survey replaces the guess with the count.
Mortar where a dry ridge should be, a valley lined with the wrong thing, a vent that was never connected to anything. None of it is your fault and all of it is in the price.
Where the scaffold can stand, whether the skip can get near the house, and whether the neighbour’s drive is involved. Scaffolding is a fixed cost and it is the line most likely to surprise you.