Before deciding on bedroom count, garage size, finishes, and specialized spaces, define the likely future buyer.
In marketing, that profile is your avatar.
Who are they? How old? Children? Income? Lifestyle? What do they value? What are they willing to pay extra for?
Start with the property and neighborhood.
Why did you buy there? Waterfront? Cheap land? Up-and-coming area? Rural lifestyle? Schools? Location?
Then ask what the area may look like five or ten years from now.
You usually do not want to build a five-million-dollar mansion in a one-million-dollar neighborhood. But if older homes are rapidly being replaced and values are rising, building slightly ahead of the market may be rational.
Sometimes a new custom home automatically becomes the most expensive house on the street. That is not necessarily bad. Somebody has to build the first one.
Define a fictional buyer in concrete terms: age range, children, income, vehicle count, work habits, entertainment style, desired bedroom count, pool expectations, office needs.
Then study real buyers. Look at sales, listings, demolitions, new construction, local businesses, schools, and neighborhood trends.
Even simple observation helps. Go to the supermarket where local residents shop and look around. It is not scientific demographic research, but it gives you a feel for the community.
Use the avatar to decide which upgrades are investments and which are personal luxuries.
Build for yourself if it is your home.
But know your exit.