If you got a LDC then applications would be residential thereafter (assuming you are a freeholder and no properties above you, as then you would then need full planning).
The answer re using PD afterwards is more complex. Most live/work units have their PD rights stripped (partly to stop change of use to wholly residential under PD), a LDC wouldn't reinstate PD rights.
First thing I would do is check if the LA actually stripped your PD rights when assigning Live/Work (they almost always do it these days but 20 years ago live/work was less common), if they did not you might be better/simpler to change use under PD rights rather than claiming it has been used historically for this use.
This would also have the benefit of no complications or retrospective action on previous use - business rates, council tax, VAT come to mind.
If you do have to go down the route of claiming it has been used historically you only need to demonstrate the minimum period (4 year) of residential use to get the LDC. One LA I can find that discuss LDC for live/work to residential is The London Borough of Hackney, they suggest you would need to prove continuous residential use for four years through such evidence as payment of Council Tax, non-payment of Business Rates, payment of other household bills, and lack of any enforcement action.
It may be difficult to prove you have done all of these to the "whole" property as:
- the LA may argue the council tax band is wrong if the whole property is residential, so whilst you can prove payment of council tax you won't been paying council tax on the portion of building that is designated "work" - you may be able to argue you have is the portion designated work is small, say 10-20%, as the banding may be unaffected, but if the property is 50% work then the council tax band will be incorrect.
- you possibly were/should have been paying business rates, if you were paying these then it will be hard to argue residential use. Again the LA will have been sending you bills, and presumably they were being paid.
Also be aware there may be VAT implication to consider on changing the use in either case.
If you are trying changing the use to use PD it may be a long shot, as possibly still won't have any PD rights, and depending on what you are trying to do you may will still need planning permission anyway - it may even be as easy to get planning under the existing use.
If you trying to regularise the use to say increase value/saleability to sell the property then it may be worth while - but a cost/valuation exercise to truly work out would perhaps be the first item to look at, and a normal application for change or use may even be easier than trying to get a LDC. If the LA resisted the application this wouldn't stop your ability to apply for a LDC thereafter either.
