The standard
BS 5837:2012, Trees in relation to design, demolition and construction, is the British Standard councils expect when trees are near a development. It asks for a survey, a tree constraints plan, and later an arboricultural impact assessment and method statement.
The categories
Each tree gets a category. A is high quality, B moderate, C low, and U is a tree that can’t realistically be retained for more than ten years. On drawings they are usually coloured: green for A, blue for B, grey for C and red for U.
The root protection area
The RPA is the area of roots a tree needs protected during building. For a single-stemmed tree it’s a circle with a radius of twelve times the stem diameter measured at 1.5 m, capped at 707 m² (a radius of about 15 m). A 60 cm oak gets a 7.2 m radius.
The RPA can be offset into a different shape where roots are known to be restricted, but that has to be justified, not assumed.
What to look at first
Open the tree constraints plan. Where the dashed RPA circles overlap your building footprint, drainage or site access, you have a design conversation to have early, when it is cheap.