Generally, if you’re buying via private treaty (not at auction) you have the ability to set some purchase conditions. For example, you may be able to make your offer ‘subject to finance’ and ‘subject to pest and building conditions’. It’s a good idea to get your solicitor or conveyancer to look into this for you and give you advice.
When you make an offer subject to conditions you need to set a date that they expire. You’ll need to factor in the time it might take for final approval to be given. You should check that time frame with your lender too.
If the house you want is being sold at auction you probably won’t be able to make a conditional offer. Auction conditions usually state you must have the deposit on the day and finance secured. Even if you want to make an offer before auction, the conditions are usually the same.