Answer: Quite certainly - yes.
Calculation example:
Market value of new house: 300,000
Equity capital: 40,000
Loan-to-value ratio bank 100 %
100% because the equity capital goes towards construction/acquisition incidental costs and a safety margin is still calculated by the bank for determining the loan-to-value ratio.
Market value of parental property: 200,000
This results in the loan value running out at around 60% and the condition decreases by about 1 percentage point.
A few more considerations:
If the parents still live in the parental house and are also the owners, they must of course participate in the land charge registration. A right of residence would have to yield in rank.
The above outlined "total" solution is very nice from the bank's perspective because there is absolutely no risk for the lender. It is also the mathematically cheapest solution because the conditions can be pressed to the market floor. However, both properties are completely "expropriated" for this. For the future, there is no scope for design anymore.
Alternative approach: The loan-to-value ratio on the new house is higher, approximately 80%. As a result, the interest rate is moderately higher. The remaining 20% is then also financed in first charge on the parental house. The loan is significantly smaller and can be repaid quickly with a high initial repayment and/or special repayments. The house becomes free of encumbrances again quickly.
It is important that both loans are treated separately and no mutual linkage occurs. This way you secure financial flexibility. And if the parents should still live in the parental house, they can probably sleep more peacefully with a small loan on their house.