Mortgage deposits for the best mortgage deals. The cheapest mortgage deals on the market will typically require you to have a 40% deposit or more, so on a £150,000 property this would mean a deposit of £60,000. However, saving up a 40% deposit is unrealistic for many people.ortgage deposits for the best mortgage deals. The cheapest mortgage deals on the market will typically require you to have a 40% deposit or more, so on a £150,000 property this would mean a deposit of £60,000. However, saving up a 40% deposit is unrealistic for many people.