Your mother can certainly claim exemption for long term capital gains arising on sale of residential plot as well as the incomplete house property under section 54 F of the income tax act. For claiming the capital gains exemption under section 54F your mother will have to invest the entire amount received on sale of these assets within two years for purchase of a new residential house or get a new residential house constructed within a period of three years. She can also book and under construction property but the construction needs to be completed within three years from date of sale of these assets. Since your mother owns only one house in addition to what she intends to buy, she is eligible to claim the tax benefits under Section 54 F.