Builders are less reliable these days. If they have a solid track record and few BBB complaints, then maybe.
You're on the right track though. The 5-year-old home will often have $5000+ in landscaping, A/C, garage finishing, etc., so be sure to consider those.
I don't you can use a set figure for price/SF/year. Some homes are built to last and are worth the same as the day they were built. I have seen 30-40 year old townhomes and condos that put the neighbors to shame in terms of construction quality.
When in doubt, talk to your local appraiser.
Hope this helped.
Helene M Moore
Windermere Prestige Properties.