Lenders are no longer happy to take all the risk of buying your next property, and so do not lend 100% of the value of the property. If you are unable, in the future, to pay your mortgage, the lender needs reassurance that it can take your home and cover the loan by selling it.
Less risk taking means lower loan-to value (LTV) ratios, and personal deposits need to be larger than in the recent past.
The source of the deposit may come from your current property, savings, inheritance or a gift. Be aware that deposit loans from family and friends can still not be accepted as a source of deposit by some lenders, or can influence how much they may lend you.