There are essentially two kinds of development – development that requires planning permission, and development that does not. If you have trees within your garden, or even within 15m of your garden, that are protected by a Tree Preservation Order or that stand within a conservation area, they will be a material consideration of any planning application that you may make.
How these trees are assessed and treated during the planning process is informed by the recommendations in BS 5837:2012 ‘Trees in relation to design, demolition and construction.’ Unless you happen to have some arboricultural qualifications, this is not a document that you can use yourself so, you are going to need some expert advice. Once this advice has been incorporated into your planning application it can proceed in the usual way.
If consent is granted, it covers you for any tree work (felling or pruning) that is immediately required for the realisation of the approved development, however, you are also likely to have conditions attached to it relating to tree protection and tree planting that you must adhere to for any trees that are being retained. The simple advice is read the conditions carefully and make sure you respect them and engage the right people to discharge them. In the future an undischarged condition may be a blight on your property.
The other kind of development is what is known as ‘permitted development’, stuff that you can build under a General Development Order. A General Development Order might be considered to be a deemed planning approval but be careful – whilst a granted full planning permission essentially ‘overrides’ a Tree Preservation Order, works undertaken under a General Development Order do not. Put simply – if you need to fell or prune a tree protected by a Tree Preservation Order to build your ‘permitted development’ structure you need specific Tree Preservation Order consent to do so and you must apply to the council to get it before you start to build anything.
With regard to the trunk and branches of the tree, the above advice ought to be pretty obvious. What you must be aware of is that Tree Preservation Orders extend to protecting the roots as well. BS 5837:2012 ‘Trees in relation to design, demolition and construction.’ gives a means of calculating the Root Protection Area of any given tree. Whilst this is not the absolute spread of the root system, the BS 5837 Root Protection Area is the zone where you must be particularly careful not to cut or damage roots anymore than you would cut or damage branches.
The upshot is that, if you have protected trees around the area you wish to go ahead with your building under a General Development Order, you still need BS 5837:2012 ‘Trees in relation to design, demolition and construction’ and you still need sound arboricultural advice.