If it commenced before 2005, I guess you might have to go through planning all over again? Don't you have five years to complete the work? Maybe it's five years to commence the work? I'd have thought you'd need a building reg guy to help out. You can get independent ones rather than having to use the local authority ones and that sounds like it might help.
I think En suite depends on how many bedrooms you have in the holiday cottage. We're just going through planning for one holiday cottage which will have one big bedroom and a sleeping loft. I insisted we have a decent bathroom - with a bath - at the expense of the kitchen because I won't rent a holiday cottage when we go away without a bath! But it won't be en suite because we didn't think that was necessary - at most it'll be two adults and two children who stay so they can share a bathroom(oh, and there wouldn't be room for an ensuite anyway).
As for open plan, I'd say it's preferable to have an open plan kitchen/living room because you often don't cook that much on holiday so it means you can have a bigger living area and if you do cook, you can chat while you're doing it.
No idea about windows - if it's listed in it's own right, you'll have to comply with listed building requirements.
I spoke to our local rep from English Country Cottages (part of the Hoseason group) and she was great with helpful pointers. She said that one of the biggest single selling point for a holiday cottage was (a little oddly) a woodburner so if you can fit one in, it's worth having (and probably extends the season of your letting potential - people imagine cosying up after autumn and winter walks). Think about your heating system too. We're just looking (again) at pellet fired boilers because the government incentives (RHI) for biofuels on commercial premises (which you'd qualify for) mean they pay back really quickly and you continue to get grants for 20 years making heating much, much cheaper. I'm also yo-yoing between underfloor heating (modern but might be set wrongly by renters) and radiators (immediate heat, easy to understand but might date in a few years). Currently going with underfloor heating but it might change tomorrow.
We also talked about who our market was, which was helpful. I'd imagined that we'd be targetting the family market - we have three small children of our own so we have the usual garden stuff for them plus acres of safe land for them to explore, animals to play with (or now geese to run away from), pretty close to beaches and family fun stuff. But the rep pointed out that if you target families, you limit your season to school holidays (or a few families with preschoolers). She said the single biggest market for holiday cottages is couples. So I've relaxed a lot more about the tiny kitchen and we'll concentrate on having one majorly lovely bedroom plus a cosy sleeping loft if families do decide to chance us. I still have a slight worry about romantic couples being disturbed by our noisy trio but we'll have to cope with that one somehow.
The other thing we've debated is whether to try and market it ourselves, go with a smaller company at a lower cut or with a big company who take a bigger cut (20% plus). Doing the sums, if a bigger company can get 30 to 40 weeks occupancy, it makes loads more sense to pay their cut than a smaller company that gets you 20 weeks. So I think we'll start with a big company and see how it goes and move to a smaller company if the occupancy is not great anyway with the big one or if we build enough of a network to market it ourselves.
If you give one of the big companies a ring, they'd probably be happy to send a rep out who can talk it all through with you.
Have fun sorting it all out!
Hester