A fixed interest mortgage is one where the interest amount remains constant throughout the loan period. Interest only mortgages are mortgages where only the interest is paid during the life of the loan. The principal becomes due at it the end of the life but commonly it is then "rolled over" into a new loan.
In valuation practice, a credit foncier loan is assumed even though the actual loan may be a fixed interest loan. Opportunity cost argues that it doesn't matter a great deal where the money comes from or how it is paid. For example, if the developer uses his/her own money instead of borrowing it, he/she will forego interest earned on that money. Therefore, all complex financial arrangements can be considered as being equivalent to the use of cash as long as the interest rates charged or foregone equal or approach market rates.