There is no evidence from the Quran or the sunnah that talks about preserving the historical sites, this is where scholars and ijtihad become really important.
The most simple example that it is permissible to expand even if it was over a historic site is the actions of some Muslim Rulers regarding this issue and the approvement of Companions/scholars from the past and the modern time . They destroyed/allowed it because this will lead to a great outcome(more muslims can pray in the mosque)
In the past: "Umar Bin Abdul Aziz" have destroyed the Hujurat(Houses where the wives of the prophet Mohamed used to stay in). They destroyed these because they needed to expand the holy mosque in Madinah. (it says that some companions didn't agree but I have to check if it is credible or not)
In our modern day: What the Saudi government is doing now with the Holy mosque in Makkah and madinah after counsel-ting the biggest clerics of Saudi Arabia
Now, somehow related. In fatwa no. (6 / 232-233), someone asked "Al-Lajna Al-Dayma" in Saudi Arabia if it is allowed to destroy an old mosque in a village because they need to build a bigger one. So they answered:
it is permissible since it will benefit many Muslims in the village. and the one who built the old mosque from his money will still receive the good deeds after building the new mosque.
Now this a mosque that has a big value in Islam than a historic site. What would you expect the answer to be ?
Now, to answer your second question, The prophet didn't say anything regarding this issue. and if we can't find an Islamic evidence from the Quran or the Sunnah then:
The very basic Fiqh rule says:
الأصل في الأشياء الإباحة
The default state of things is permissible
A simple example, We still have many historical sites such as Muslim and non Muslim fortresses or other ancient places.
However the prophet asked Muslims to destroy these:
In the hadtih:
Abu'l-Hayyaj al-Asadi told: that 'Ali (b. Abu Talib) said to him:
Should I not send you on the same mission as Allah's Messenger (may peace be upon him) sent me? Do not leave an image without obliterating it, or a high grave without levelling It. This hadith has been reported by Habib with the same chain of transmitters and he said: (Do not leave) a picture without obliterating it.
The previous hadith talks about mashhads (shrines) or tombs or mausoleums which should be destroyed
Urwah ibn ‘Abasah said to the Prophet (peace and blessings of Allaah be upon him):
"With what were you sent?" He said, "I was sent to uphold the ties of kinship, to break the idols, and so that Allaah would be worshipped alone with no partner or associate."
Also, The Prophet (peace and blessings of Allaah be upon him) sent Khaalid ibn al-Waleed (may Allaah be pleased with him) on a campaign to destroy al-‘Uzza(idol people used to worship).
And he sent Sa’d ibn Zayd al-Ashhali (may Allaah be pleased with him) on a campaign to destroy Manaat(another idol people used to worship).
And he sent ‘Amr ibn al-‘Aas (may Allaah be pleased with him) on a campaign to destroy Suwaa’(another idol). All of that happened after the Conquest of Makkah.
Al-Bidaayah wa’l-Nihaayah, 4/712. 776. 5/83; al-Seerah al-Nabawiyyah by Dr. ‘Ali al-Salaabi, 2/1186.
Off topic :
*Q1-*What about the verse in the Quran:
وكذلك أعثرنا عليهم ليعلموا أن وعد الله حق وأن الساعة لا ريب فيها إذ يتنازعون بينهم أمرهم فقالوا ابنوا عليهم بنيانا ربهم أعلم بهم قال الذين غلبوا على أمرهم لنتخذن عليهم مسجدا
And similarly, We caused them to be found that they [who found them] would know that the promise of Allah is truth and that of the Hour there is no doubt. [That was] when they disputed among themselves about their affair and [then] said, "Construct over them a structure. Their Lord is most knowing about them." Said those who prevailed in the matter, "We will surely take [for ourselves] over them a masjid."
The verse clearly mentions that the Quran mentioned that people built a mosque over a grave ? Doesn't that mean we should follow the Quran ?
Answer: The Quran didn't say if Allah has approved what those did. The Quran basically mentioned their story and what they did. So we need to look at the hadith. and we have seen that the prophet has ordered the companions to level any high grave.
Also, Ibn Kathir commented on this verse, He said:
(those who won their point said: "We verily, shall build a place of worship over them.'') Those who said this were the people of power and influence, but were they good people or not There is some debate on this point, because the Prophet said:
«لَعَنَ اللهُ الْيَهُودَ وَالنَّصَارَى اتَّخَذُوا قُبُورَ أَنْبِيَائِهِمْ وَصَالِحِيهِمْ مَسَاجِد»
(Allah has cursed the Jews and the Christians who took the graves of their Prophets and righteous people as places of worship) Warning against what they did. We have reported about the Commander of the faithful `Umar bin Al-Khattab that when he found the grave of Danyal (Daniel) in Iraq during his period of rule, he gave orders that news of this grave should be withheld from the people, and that the inscription containing mention of battles etc., that they found there should be buried.
*Q2-*But what about the idols in the Pyramids or statues in other places were people used to worship? Why didn't the companions destroy the idols inside these places ?
Answer: Scholars said :
1st reason, it was remote, so they couldn't reach it. Or the 2nd reason it wasn't visible and they didn't go inside an example is the pyramid they didn't go inside because of the hadith:
The Prophet (PBUH) told us to hasten when passing through the abodes of the wrongdoers and those who had been punished, and he forbade entering such places.
Another hadith In Bukhary and Muslim says:
Do not enter upon those who have been punished unless you are weeping, lest there befall you something like that which befall them." He (peace and blessings of Allaah be upon him) said that when he passed by ashaab al-hijr [the dwellers of the rocky tract – see al-Hijr 15:80], in the land of Thamood, the people of Saalih (PBUH).
Another narration that prohibited them is:
If you are not weeping, then do not enter upon them, lest there befall you something like that which befall them.
Bukhary and Muslim
What we(scholars) think is that if the companions of the Prophet (PBUH) saw a temple or building belonging of these people, they did not enter it or even look at what was inside it.
the 3rd reason, Many of these idols that are visible nowadays were covered and hidden, and have only been discovered recently, or they have been brought from remote places that the companions of the Prophet (PBUH) did not reach.
Al-Zarkali was asked about the Pyramids and the Sphinx etc: Did the companions who entered Egypt see them?
They were mostly covered with sand, especially the Sphinx.
Shibh Jazeerat al-‘Arab, 4/1188
And another Muslim ruler tried destroying a historic statue but he couldn't do that.
Ibn Khuldoon said in al-Muqaddimah (p. 383), that
the caliph al-Rasheed was unable to destroy the estrade of Chosroes. He started to do that, and he gathered men and tools, and burned it with fire, and poured vinegar on it, but he was unable to do it. And the caliph al-Ma’moon wanted to destroy the Pyramids in Egypt and he gathered workers but he could not do it.
As you noticed from above that historic sites that have idols or may lead to shirk and Kufur
The third question, There are some hadiths that show it is not allowed to adorn the mosques and exaggerate in decorating them. and the least thing scholars have said regarding this issue is that it is Makrooh(disliked or offensive act) to adron mosques.
Narrated Abdullah ibn Abbas:
I was not commanded to build high mosques. Ibn Abbas said: You will certainly adorn them as the Jews and Christians did.
And the Buhkary commented in his book on (1 / 171) that when Umar(RA) asked to improve the mosque of the prophet in Madinah, He said:
Add something that would protect the people from rain, and don't use a red or yellow color(referring to decorating and adorning the mosque) and distract them by it.
Source of the hadith
What about destroying them ? If you are asking should we destroy them because we need to expand the place ? then it is answered above.
What about destroying them because they exhibit a lot of archaeological features and beauty? Then I don't know the answer to that. Sorry!!
EDIT: From this Fatwa it says that some scholars from Hanbali and one of the sides of Shafii said that you should remove the gold and silver if they have been used in decorating a mosque because it is considered waste, and make poor people sad because instead of giving them money you are wasting it on decoration.
To answer your question in the comments "does golden mausoleums also come under this ruling to be demolished?"
If gold has been used then as I mentioned earlier, Gold should be removed according to the Hanbali, and Shafi school of thought.
If I have made any mistake it is from me, If it was correct then it is from Allah, May allah forgive me if I issued a fatwa. If you guys see anything wrong please comment so I would know.