Assuming the following sunario
Land 100,000
Cost of building house 150,000
Legal Expenses 3,000
Sale of house 403,000
then the profit is 150,000
CGT is then calculated as follows
Profit (Capital Gain) 150,000
Allowance 1,270
Liable to tax 148,730
Tax @ 20% 29,746
Assuming your original house is your PPR and is owned by you over 5 years you could rent it out and move into your new built house. This then becomes your PPR.
If your current home is owned by you for less than 5 years and was bought as a first time buyer i.e no stamp duty or reduced stamp duty you will be caught for stamp duty clawback and will have to pay the revenue the stamp duty applicable to the investors rate.
You will have to make tax returns for the rental income less allowable expenses to revenue every year