The Society of Garden Designers (SGD) chose the third-year undergraduate student, who is studying for a BA Hons degree in garden design at University College Falmouth.
An SGD representative said: "Maren's response was comprehensive and innovative. She was skilled in dealing with a complex site and design software."
The representative added: "She showed a highly informed understanding of the environmental and visual context, which informed the proposal."
Course leader Richard Sneesby said the design reflected a thoughtful and detailed approach.
Hallenga said: "Coming first was a great confidence boost and will hopefully set me up for a successful future in garden design."
The award is made annually and is open to the society's 400 student members.