54f

it can be before the sale of property.  Enough case laws available on this issue.  Kindly do some research....

Agreed. Construction may commence even before transfer of capital asset. However, the construction should be completed within 3 years of transfer. Date of commencement of construction is irrelevant. You can refer to case law: CIT v. Bharti Mishra[2014]

Construction of another house property may start before the sale of the house property in question, but it should be completed within the specified time of claiming exemption u/s 54F. For your clarity, I am stating the law in a bit simplified form. For bare text, kindly refer https://www.incometaxindia.gov.in/pages/acts/income-tax-act.aspx

U/s 54F, any long term capital gains arising to any individual of HUF from sale of residential house property(whether self occupied or on rent), shall be exempt to the extent such capital gain is invested in:

• Purchase of another Residential House property within one year before or two years after the due date of transfer of  the property sold and/or;
• Construction of residential house property within a period of 3 years from the date of acquisition.

The New House Property must be held as capital asset for 3 years from the date of purchase or construction.

Relevant case laws- CIT vs Bharti Mishra, UOI vs Wood Papers Limited, G.P. Ceramics Pvt Ltd. vs CTT

FOR THE PURPOSE OF CLAIMING EXEMPTION U/S 54F, CONSTRUCTION OF NEW PROPERTY HAS TO BE COMPLETED WITHIN 3 YEARS FROM THE DATE OF SALE. THERE IS NO CONDITION THAT CONSTRUCTION SHOULD ALSO BE STARTED AFTER SALE. IT CAN START BEFORE THE SALE AS WELL.

FURTHER IF YOU HAVE SOLD A HOUSE PROPERTY AND INVEST THE GAINS IN CONSTRUCTION OF NEW PROPERTY THEN WHY HAVEN'T YOU CLAIMED EXEMPTION U/S 54 INSTEAD OF 54F ?