A lot of people are finding it difficult to sell their house with plenty of houses on the market for sale for over a year. In the current market I would advise you to sell your own house first and then use some of the cash from your house towards the new house. You might be lucky and sell your housily easily for above what you are expecting, however, it's likely you won't get as much as you want. If you take out the 40k mortgage or increase your existing mortgage by 40k you are going to start feeling the pressure when your house is on the market and you're not getting any offers in.