My older two are adults (younger than @TiredAndBurntOutthough). We've always told them that we'll take no board until they finish their first post 18 education, whatever that is, but will expect the equivalent of child benefit from them after that, or if they choose to drop out in which case they need to work obviously (unless there's a health issue in which case everything is different). We also set 25 at the age from which they're full blown adults and we'd expect to be splitting all costs by the number of people in the house and everyone over 25 paying their share, with us parents covering under 25s.
For us that all makes sense and some of it has backstory which makes it make sense to our family, and of course there would in reality be exceptions possible...
However for a 27 year old without disabling health issues or other extenuating circumstances I'd expect all housing bills to be split equally, and the younger adult to have equal oversight of the costs - to know exactly what running the house costs, what needs paying when and how much it usually is, which energy suppliers we use etc. and to be involved in decisions such as deciding to change supplier, invest in something - better insulation, a new boiler, and have an adult and reasonable discussion about whether it's appropriate to pay a share.
Running a home long term involves a steing of bills but also long term savings for one off costs - if you own you should be putting money into a home maintenance "pot" each month for big repairs not covered by home insurance (roof, boiler, plumbing but also repainting externally and internally every 5 years or so - and things like ovens and fridges and washing machines tend to need replacing every ten years or so, at some point, maybe every 15 years, carpets if you have them need replacing - at 27 you've been using all the facilities of your parent's house for nine years as an adult so there's the question of when you consider those your expenses too - I'd say set a timeline for moving out or start paying 50/50 if you're staying). Bills aren't limited to rent/ mortgage, heating, light/ electricity and streaming services, there's also home and contents insurance, council tax, water etc etc.
27 is very different to 18 or 21 - it's time for an adult conversation as equals and taking equal responsibility (in return for parental transparency and clarity about costs and treating you as an equal going forward) OR alternatively you have the adult conversation about your maximum twelve month timeline to move out and request another twelve months of financial support to allow you to save hard, and agree between you what that means in terms of limiting your financial input into your current household.
My kids will always be welcome but I don't think treating them like 20 year olds indefinitely would do them any favours - if they're still at home at 27 we'll be splitting everything, but they'll have full understanding of the household finances (assuming they're not living with us due to a health or personal acute crisis).
Good luck!