BiggerPockets also has a calculator to analyze deals, and I highly recommend you start this as soon as possible, even if you are not ready to buy.

Roughly calculate €1500 for Spanish residents and €2500 for foreigners (also having in mind Power of Attorneys to delegate some of the work so you don't need to be here yourself) to set up the SL.

I appreciate that, and you're right, any back-of-the-envelope calculation suggests that much of the wealth comes from the equity accumulation, even in the earlier stages of the amortization schedule.

Usually my agent is the one asking me to how to calculate ARV and find cost to fix the bathroom,etc.

That's because the city won't calculate the actual amount till a structure is built, aka it's not just a plot of land anymore.This will obvious cause your property tax bill to go up and also your mortgage escrow account as well (sometimes these things can make your mortgage shoot up a few hundred a month). 

If you are planning on living there (ie in 1 unit) then you are house-hacking and this is a different calculation consideration then renting out both units and making it a true investment.

Even if you want to roll the dice, at least is a calculated risk, not a surprise down the road. 

They calculate a "net effective" rent that's more appealing to the tenant, basically amortizing the concession over the lifetime of the lease.

BiggerPockets also has a calculator to analyze deals, and I highly recommend you start this as soon as possible, even if you are not ready to buy.

BiggerPockets also has a calculator to analyze deals, and I highly recommend you start this as soon as possible, even if you are not ready to buy.