David Cameron has written to the National Trust reassuring them over plans for shaking up the planning system.
The Prime Minister said new laws were designed to "achieve a balance" between the environmental, social and economic dimensions of developments.
He said the "beautiful British landscape is a national treasure" and expressed determination to protect the "magnificent countryside".
"Poorly designed and poorly located development is in no one's interest," Mr Cameron wrote.
The Trust has been leading a campaign to oppose the reforms, which are currently the subject of consultation.
Mr Cameron's letter, seen by The Daily Telegraph, reads: "Let me say at the outset that I absolutely share and admire your commitment to the countryside, and wholeheartedly agree that policymakers have an enormous responsibility to our environment.
"Both as Prime Minister, as a rural constituency MP, and as an individual, I have always believed that our beautiful British landscape is a national treasure.
"We should cherish and protect it for everyone's benefit."
Ministers are pushing through plans to replace more than 1,000 pages of planning regulations with just 52 in the National Planning Policy Framework.
The change is controversial because it writes into the rules a "presumption in favour of sustainable development".
Campaigners fear swathes of England could be concreted over as urban sprawl gathers pace.
Mr Cameron insisted that sustainable development would be defined to include a reference to the environment and social impact of proposed projects.
He wrote: "Our reform proposals are intended to simplify the system, strengthen local participation and secure sustainable development.
"I believe that sustainable development has environmental and social dimensions as well as an economic dimension, and we fully recognise the need for a balance between the three.
"Indeed, the purpose of the planning system as a whole, and of our proposals for it, is to achieve such a balance."
Mr Cameron added: "We must ensure the appropriate protections for our magnificent countryside.
"This is why our reforms will maintain protections for the green belt, for National Parks and Areas of Outstanding National Beauty.
"It will introduce a new local green space designation which local communities can use to protect open places they value."
However, Mr Cameron also warns campaigners that new developments are essential to boost Britain's stalling economy. He said that businesses should be able to expand and that the difficulty encountered by first-time buyers of getting on to the property ladder was "unacceptable".
The Trust welcomed Mr Cameron's comments.
Fiona Reynolds, its director general, said: "Our primary concern for the planning system is that it should be a neutral framework which balances the needs of society, the environment, as well as the economy.
"It is a great relief to hear from the Prime Minister that there is no intention to change this over-riding purpose.
"We will now do everything in our power to help shape the National Planning Policy Framework into a robust system which enables the people making the decisions to guide good development to the right places. Planning Minister Greg Clark has invited us to work with him to ensure the detail of the document reflects this brief, and we are keen to do so."