Watford Borough Council puts the protection in one line: "Some trees in the borough are protected by a TPO or are protected by being in a Conservation Area. Bushes and shrubs are not covered", and "A TPO or Conservation Area makes it an offence to cut down, lop, prune, top, uproot, deliberately damage or destroy a tree without written consent from us." On its conservation areas page it adds "You must give notice to do work to trees over a certain size in a conservation area", with applications through the Planning Portal. Its Conservation Areas Management Plan says "Within the Borough of Watford there are ten areas which have been designated as conservation areas".
The council does not state the notice period on either page. Nationally, gov.uk describes the requirement for "people to notify the local planning authority, using a section 211 notice, 6 weeks before carrying out certain work on such trees, unless an exception applies", and says the work may go ahead sooner where the authority gives consent. The same guidance states that consent "is needed for carrying out work on diseased and/or dying trees unless some other exemption applies".
On felling, the Forestry Commission service states that a licence is not required for trees in private gardens, orchards, churchyards, or public open spaces like some parks, and that up to 5 cubic metres can be cut from a property in a calendar quarter without one, of which no more than 2 may be sold.
We would explain how you check the conservation area boundary, any tree preservation order and the proposed work before confirming whether a section 211 notice or consent is needed.