Assuming that you are buying in England (the rules differ in Scotland), the house is not yours until contracts have been signed, monies exchanged and the deal completed. You, and others in the chain, will have incurred costs that are unlikely to be recovered if the sales do not go through, so it is in everyones' interest if the sales are completed.
Often when making an offer, it is stated that the offer is only made on the understanding that the sale will be completed within 8 weeks (or whatever). This makes your intentions clear from the start but it does not guarantee that the sale will go through.
If you make an offer to buy anything other than a house, and the offer is accepted, that offer is legally binding. With houses in England, however, very little is binding until the contracts are signed.