Meeting deadlines is what separates #professionals from #amateurs. To accept a #deadline that you know beforehand will be impossible to meet, aiming to please the client and hoping to deal with the #problem later with some ingenious #negotiation is one of those decisions you will regret. You may lose a client and gain a bad reputation in the process.
Being trustworthy is essential for best practice, especially if you're a freelancer. Saying no to impossible deadlines, or that you won't be able to meet, doesn't tell your client you are lazy or too slow, quite the opposite; The #client is reassured that every #project you take on won't be dealt with in a #pressing manner and will be thoroughly reviewed because you had adequate time to do it.
Negotiating an acceptable deadline for both parties is a skill you need to use if you want to succeed. You are not the only person capable of doing the job but you can rise above many #competitors by doing the right thing. Assess the workload and give the client the opportunity to give the project to someone else if needs be.