Adding house value is different to adding space. Space, i.e. more square metres generally adds at least the cost of the works, but that’s just building, its not design or architecture and rarely adds value.
Defining value is pretty conceptual, but when we see something that works, we know it has value. Value is one of those intangibles, precious to some irrelevant to others, we make value choices every day, food, clothes and how and where we spend our time and money. Value can be elusive or obvious!