President Reagan said this week he wants to rewrite the rules for real estate tax write-offs, simultaneously simplifying the system and speeding up the rate at which investments can be recovered through depreciation deductions.

The changes proposed by the new administration are meant to encourage construction of factories, stores and rental housing by increasing the cash flow and profitability of real estate investments.

Equally important, what Reagan calls "the accelerated cost recovery system" will introduce elements of certainty and order to the often chaotic rules of real estate depreciation.

At present, investments in income-producing real estate can be depreciated over periods ranging from 25 to 60 years, with accelerated depreciation for certain kinds of property.

Every property is looked at individually and the general rule is that you can claim whatever depreciation the Internal Revenue Service will let you get away with.

Tax lawyers, accountants and property management specialists make their living developing depreciation plans and defending them to the tax collectors.

Reagan has asked Congress to replace the complications and compromises with a fixed set of what are called "audit-proof" depreciation schedules that call for:

Ten-year write-offs for owner-occupied factories, stores and warehouses;

Fifteen-year depreciation for business and commercial property not used by the owner and for low-income rental housing;

Eighteen-year depreciation for other residential income property.

All leased property would have to be depreciated on a straight-line basis, in which the cost of the property is reclaimed in equal installments, 1/15th or 1/18th every year.

Owner-occupied factories, stores and warehouses would be eligible for the same kinds of accelerated write-offs now available, the administration's summary of its plan indicates.

However, the investment tax credits now offered to encourage certain kinds of real estate investments would be eliminated under the Reagan plan.

Details of the real estate depreciation proposal have not all been announced since Reagan's Thursday night speech. The administration still has to persuade Congress to pass the depreciation package, but there is considerable sentiment on Capitol Hill for both the simplification and substance of the plan.

The National Apartment Association immediately praised the provisions for quicker write-offs on rental housing, but warned that the other parts of the package might drain real estate investment away from housing.

"By providing faster write-offs, increased amounts of capital will be invested in the productions of new housing, thus allievating the rental housing shortage facing this nation," said Stan Taube, president of the apartment owners and builders group.

Apartment buildings currently can be depreciated over periods ranging from 30 to 40 years. Accelerated depreciation of up to 125 percent of usual amount can be claimed on used residential property; low-income housing also gets favored treatment.

Taube praised provisions of the Reagan plan establishing lives of 15 and 18 years for rental residential property but complained that "by providing faster write-offs for investments in owner-occupied commercial and industrial buildings, capital will be attracted away from residential housing, defeating the purpose of faster write-offs for residential rental property."