A simple door will be just fine ;) I don't really know what style to go for, as it needs to let light through in the day, without looking old fashioned

You need to post pictures for the best advice

Thanks for all your comments. Personally I do not need extra space for the kitchen and would prefer the door in the kitchen for convenience especially to dispose the rubbish/recycling materials outside into the big wheelie bins located next to the door outside. The space under the stairs will housed the boiler and general/recycling bin. Due to utility space constraint I prefer no doors and to keep the existing window where the sun gets into. I intend to dry my clothes in the utility room next to the window or using the dryer hence do not feel the need to have access to outside. That is because I do not have dogs or an outdoor person coming in with muddy boots. I am glad that most have said it is personal choice rather than a norm. This is because I am also looking to ensure that any work done to the house will be saleable in the future or appeal to a wider population.

Modifying an existing patio usually falls under permitted development, so planning permission shouldn't be needed. However, re: building regulations there are rules around step height, depth and handrails (needed if the total rise is more than 600mm, about two steps). The tricky bit might be the bifold doors. Building regs might require a minimum space to step out comfortably. If the steps are too shallow near the doors, they could be an issue. Also when you are designing the steps, make sure they're deep enough to step out safely while still meeting the regulations. I would always suggest chatting with a building inspector or a builder who knows their regulations first before doing anything.