The Commissioner shall require assurance of completion of off-site public utilities and streets in all cases, except where a municipality or other public body has by agreement acceptable to HUD agreed to install such utilities and streets without cost to the mortgagor. Where such assurance is required, it shall be in the form of a cash escrow deposit, a letter of credit, the retention of a specified amount of mortgage proceeds by the mortgagee, or a combination thereof. In any case, the amount of deposit or retained cash (or both) must be sufficient to cover the cost of off-site utilities and streets. If a cash escrow is used, it shall be deposited with the mortgagee or with an acceptable trustee or escrow agent designated by the mortgagee. If mortgage proceeds are used, the mortgagee shall retain under terms approved by HUD, rather than disburse at the initial closing of the mortgage, a sufficient portion of the mortgage proceeds allocated to land in the project analysis. As additional assurance, HUD may also require a surety company bond or bonds.