What does it mean to 'extend one's house'?

[Kirijji]

Business Idiom in English, Intermediate level

ESL/EFL Test #131 "Ways of approving", question 6

The local council has approved the plans for ......... their house.

(a) developing
(b) lengthening
(c) widening
(d) extending

Business Idiom in English, Intermediate level

ESL/EFL Test #131 "Ways of approving", answer 6

The local council has approved the plans for extending their house.

Correct answer: (d) extending

Your answer was: incorrect
The local council has approved the plans for developing their house.
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What does it mean to 'extend one's house'?

Thank you!
Kirijji

[Conchita]

To extend a house is to make it bigger.