What's the problem?
For a housing development to happen, developers have to show they will not cause any extra runoff.
Council's flood map overestimates current runoff. This means replacing pine forest with roads and roofs in the future looks like it won't increase runoff.
But in reality it could increase by as much as ...%.
This would increase flooding risk and raise house insurance prices, which would all fall on the shoulders of locals.
Before any development can happen we need to know what the real increase in flood risk would be, and that Guildford developers will correct for it.
Why is the current flood map wrong?
Treats pine forest as impermeable rock when really lots of rain is absorbed into the hills. This means they overestimate how much rainfall runoff is reaching the valley floor and possibly flooding.