The couple wants to build a single-story house initially due to their budget, but add a second story in the future as their family grows. Key considerations for the initial design include ensuring the foundation can support future additions, planning for at least two bedrooms initially with potential for more, and allowing future space for things like a pool. The long-term plan over decades includes adding a second story, accommodating aging parents, and preparing for the house to be used until retirement or sold as an investment.