<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>2020 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>D2013Q2YTD</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000355429</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>us-gaap_AccountingPoliciesAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>pl_FixedIndexedAnnuityProductsPolicyTextBlock</ElementName><ElementPrefix>pl_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="D2013Q2YTD" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-size:10.0pt;font-family:Times New Roman;"&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;Investment Products&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 37pt; MARGIN: 0in 0in 0pt;"&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt;" size="2"&gt;The Company establishes liabilities for fixed indexed annuity (&amp;#8220;FIA&amp;#8221;) products.&amp;#160;These products are deferred fixed annuities with a guaranteed minimum interest rate plus a contingent return based on equity market performance.&amp;#160;The FIA product is considered a hybrid financial instrument under the Financial Accounting Standards Board (&amp;#8220;FASB&amp;#8221;) Accounting Standards Codification (&amp;#8220;ASC&amp;#8221; or &amp;#8220;Codification&amp;#8221;) Topic 815 &amp;#8212; &lt;i&gt;Derivatives and Hedging&lt;/i&gt; which allows the Company to make the election to value the liabilities of these FIA products at fair value. This election was made for the FIA products issued prior to 2010 as the policies were issued.&amp;#160;These products are no longer being marketed. The changes in the fair value of the liability for these FIA products are recorded in &lt;i&gt;Benefit and settlement expenses&lt;/i&gt; with the liability being recorded in &lt;i&gt;Annuity account balances&lt;/i&gt;. For more information regarding the determination of fair value of annuity account balances please refer to Note 13, &lt;i&gt;Fair Value of Financial Instruments&lt;/i&gt;. &lt;i&gt;Premiums and policy fees&lt;/i&gt; for these FIA products consist of fees that have been assessed against the policy account balances for surrenders. Such fees are recognized when assessed and earned.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 34pt; MARGIN: 0in 0in 0pt;"&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt;" size="2"&gt;During 2013, the Company began marketing a new FIA product.&amp;#160;These products are also deferred fixed annuities with a guaranteed minimum interest rate plus a contingent return based on equity market performance and are considered hybrid financial instruments under the FASB&amp;#8217;s ASC Topic 815 &amp;#8212; &lt;i&gt;Derivatives and Hedging&lt;/i&gt;.&amp;#160;&amp;#160; The Company did not elect to value these FIA products at fair value, as a result the Company accounts for the provision that provides for a contingent return based on equity market performance as an embedded derivative.&amp;#160;The embedded derivative is bifurcated from the host contract and recorded at fair value in &lt;i&gt;Other liabilities&lt;/i&gt;.&amp;#160;Changes in the embedded derivative are recorded in &lt;i&gt;Realized investment gains (losses) &amp;#8212; Derivative financial instruments&lt;/i&gt;. For more information regarding the determination of fair value of the FIA embedded derivative refer to Note 13, &lt;i&gt;Fair Value of Financial Instruments&lt;/i&gt;. The host contract is accounted for as a debt instrument in accordance with ASC Topic 944 &amp;#8212; &amp;#160;&lt;i&gt;Financial Services &amp;#8212; Insurance&lt;/i&gt; and is recorded in &lt;i&gt;Annuity account balances&lt;/i&gt; with any discount to the minimum account value being accreted using the effective yield method.&amp;#160; &lt;i&gt;Benefits and settlement expenses&lt;/i&gt; include accreted interest and benefit claims incurred during the period.&lt;/font&gt;&lt;/p&gt;
&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>Disclosure of accounting policy for various fixed indexed annuity products of the entity.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Investment Products</Label></Row><Row FlagID="0"><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>pl_NewAccountingPronouncementsAdoptedPolicyTextBlock</ElementName><ElementPrefix>pl_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="D2013Q2YTD" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-size:10.0pt;font-family:Times New Roman;"&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;Accounting Pronouncements Recently Adopted&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;ASU No.&amp;#160;2011-11&amp;#8212;Balance Sheet&amp;#8212;Disclosures about Offsetting Assets and Liabilities.&lt;/font&gt;&lt;/b&gt;&lt;font style="FONT-SIZE: 10pt;" size="2"&gt;&amp;#160; This Update contains new disclosure requirements regarding the nature of an entity&amp;#8217;s rights of offset and related arrangements associated with its financial and derivative instruments. The new disclosures are designed to make financial statements that are prepared under GAAP more comparable to those prepared under IFRSs. Generally, it is more difficult to qualify for offsetting under IFRSs than it is under GAAP. As a result, entities with significant financial instrument and derivative portfolios that report under IFRSs typically present positions on their balance sheets that are significantly larger than those of entities with similarly sized portfolios whose financial statements are prepared in accordance with GAAP. To facilitate comparison between financial statements prepared under GAAP and IFRSs, the new disclosures will give financial statement users information about both gross and net exposures. In January&amp;#160;2013, the FASB issued ASU No.&amp;#160;2013-01, which clarifies that application of ASU No.&amp;#160;2011-11 is limited to certain derivatives, repurchase and reverse repurchase agreements, and securities borrowing and lending transactions. Both Updates were effective January&amp;#160;1, 2013. Neither Update had an impact on the Company&amp;#8217;s results of operations or financial position.&lt;/font&gt;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;ASU No.&amp;#160;2012-02&amp;#8212;Intangibles&amp;#8212;Goodwill and Other&amp;#8212;Testing Indefinite-Lived Intangible Assets for Impairment.&lt;/font&gt;&lt;/b&gt;&lt;font style="FONT-SIZE: 10pt;" size="2"&gt;&amp;#160; This Update is intended to reduce the complexity and cost of performing an impairment test for indefinite-lived intangible assets by allowing an entity the option to make a qualitative evaluation about the likelihood of impairment prior to the quantitative calculation required by current guidance. Under the amendments to Topic 350, an entity has the option to first assess qualitative factors to determine whether it is more likely than not that an indefinite-lived intangible asset is impaired as a basis for determining whether it is necessary to perform the quantitative impairment test. If an entity determines it is not more likely than not that impairment exists, quantitative impairment testing is not required. However, if an entity concludes otherwise, the impairment test outlined in current guidance is required to be completed. The Update does not change the current requirement that indefinite-lived intangible assets be reviewed for impairment at least annually. This Update was effective January&amp;#160;1, 2013. This Update did not have an impact on the Company&amp;#8217;s results of operations or financial position.&lt;/font&gt;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;ASU No.&amp;#160;2013-02&amp;#8212;Comprehensive Income&amp;#8212;Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income.&lt;/font&gt;&lt;/b&gt; &lt;font style="FONT-SIZE: 10pt;" size="2"&gt;The amendments in this Update supersede the presentation requirements for reclassifications out of accumulated other comprehensive income in ASU No.&amp;#160;2011-05, Comprehensive Income&amp;#8212;Presentation of Comprehensive Income, and ASU No.&amp;#160;2011-12, Comprehensive Income&amp;#8212;Deferral of the Effective Date for Amendments to the Presentation of Reclassifications of Items Out of Accumulated Other Comprehensive Income in Accounting Standards Update No.&amp;#160;2011-05, for all entities. The amendments do not change the current requirements for reporting net income or other comprehensive income in financial statements. The Update requires an entity to report the effect of significant reclassifications out of accumulated other comprehensive income on the respective line items in net income if the amount being reclassified is required under GAAP to be reclassified in its entirety to net income. For other amounts that are not required under GAAP to be reclassified in their entirety in the same reporting period, an entity is required to cross-reference other disclosures required under GAAP that provide additional detail about those amounts. The Company has added the Accumulated Other Comprehensive Income footnote to disclose the required information beginning in the first quarter of 2013. This Update was effective January&amp;#160;1, 2013. This Update did not have an impact on the Company&amp;#8217;s results of operations or financial position.&lt;/font&gt;&lt;/p&gt;
&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>Disclosure of accounting policy pertaining to recently adopted accounting pronouncements.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Accounting Pronouncements Recently Adopted</Label></Row><Row FlagID="0"><Id>4</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>pl_NewAccountingPronouncementsNotYetAdoptedPolicyTextBlock</ElementName><ElementPrefix>pl_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="D2013Q2YTD" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-size:10.0pt;font-family:Times New Roman;"&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;Accounting Pronouncements Not Yet Adopted&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;ASU No.&amp;#160;2013-10&amp;#8212;Derivatives and Hedging&amp;#8212;Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes.&lt;/font&gt;&lt;/b&gt; &lt;font style="FONT-SIZE: 10pt;" size="2"&gt;This Update provides for the inclusion of the Fed Funds Effective Swap Rate (&amp;#8220;OIS&amp;#8221;) as a U.S. benchmark interest rate for hedge accounting purposes, in addition to U.S. Treasury rates and LIBOR. The amendments in the Update also remove the restriction on using different benchmark rates for similar hedges. The amendments are effective prospectively for transactions entered into on or after July&amp;#160;17, 2013. The Company is currently evaluating the impact of the Update on its policies and processes.&lt;/font&gt;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&amp;#160;&lt;/p&gt;
&lt;p style="TEXT-INDENT: 0.5in; MARGIN: 0in 0in 0pt;"&gt;&lt;b&gt;&lt;font style="FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold;" size="2"&gt;ASU No.&amp;#160;2013-11 &amp;#8212; Income Taxes &amp;#8212; Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists.&lt;/font&gt;&lt;/b&gt;&lt;font style="FONT-SIZE: 10pt;" size="2"&gt;&amp;#160; The objective of this Update is to eliminate diversity in practice related to the presentation of certain unrecognized tax benefits.&amp;#160;The Update provides that unrecognized tax benefits should be presented as a reduction of a deferred tax asset for a net operating loss or other tax credit carryforward when settlement in this manner is available under the tax law.&amp;#160;The amendments are effective for annual periods beginning after December&amp;#160;15, 2013 and interim periods therein.&amp;#160;The Update does not require new recurring disclosures, and is not expected to have an impact on the Company&amp;#8217;s results of operations or financial position.&lt;/font&gt;&lt;/p&gt;
&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>Disclosure of accounting policy pertaining to accounting pronouncements not yet adopted.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Accounting Pronouncements Not Yet Adopted</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://www.protective.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>4</NumberOfRows></InstanceReport>
