The "compromis" or promise is the first step in buying a property in France, whether it is land or buildings. Until the compromis has been signed you have no deal with the seller and it is for this reason that the compromis is frequently signed at the time the offer is made. But the buyer should be aware that the compromis is a legally binding contract which can oblige you, as buyer, to proceed with the purchase.

Like any legal document, you can add in clauses specific to your situation but many estate agents use a pre-printed compromis which only foresees the more common terms.

If the compromis is not signed in front of a notaire there is a period of 7 days during which you can retract but this can be quite difficult to do from overseas as it will require sending a certified-delivery letter to each party. For this reason alone you should NEVER back-date a compromis. If the compromis is signed before a notaire this 7 day period is not obligatory.

Provided buyer and seller are in agreement you can add in clauses which make the compromis conditional (or "suspensif" in French). Commonly such a clause is added if your purchase requires a loan and even if you don't need a  loan it is perhaps best to request such a clause in order to give you a get-out later - the seller is not obliged to accept such a clause but nearly always will.

It is not uncommon for the purchase of land to include a clause making the purchase conditional on obtaining planning permission but such a clause would have to be realistic in terms of what permission you would ask for.

Less common with an existing property is making the purchase conditional on getting permission for an extension or a pool but again it is something for buyer and seller to agree.

Whatever conditions are agreed there must be a time-limit after which the compromis lapses.

Normally a 10% deposit is paid at the time a compromis is signed and although in theory it would be forfeit if you simply changed your mind and pulled out of the purchase, in practice the compromis remains in force until is it renounced by all parties. As long as the compromis is still in force the buyer can't get his deposit back but neither can the seller proceed with another buyer - in other words it is negotiable.